Notes on the Mount Lyell District

NOTES

ON THE

MOUNT

LYELL

DISTRICT,

TASMANIA.

By

Frederick

Danvers

Power,

F.G-.S.

(Corresponding

Member

Royal SocietyofTasmania.)

On

taking a casual glance at thegeologically coloured

map

of

Tasmania we

arestruckwith its similarity inappearanceto

some huge

concretion, for

we

see a nucleus of greenstone,

whichis surrounded

more

orlessby a ring oftheupper coal

measures,

and

these againby rocks fromotherepochs.

If insteadof the geological

map

we

take a topographical

one

we

will observe the

same

onion-like structure. Inthe

centre

we

find the

Lake

Plateau country, from which the

river systems of

Tasmania

radiate. Encircling this are

various ranges of mountains,

and

finally

we

have the sea

coast, which has practicallythe

same

contour as thekernel,

making

due allowance for irregularities, which

would

be

exaggerated by enlargements. Looking at the island as a

whole, it isheart-shaped,

and

somewhat

similar in outline to

the continents, inasmuchasit iswidest at the northern

and

narrowest at its southern end,its length being north

and

south.

The

geological

and

topographical features coinciding

so closely,

we

are naturally led to the conclusion that they

must

in

some

way

or otherbe intimately related. If

we

can

furtherconnect ourore deposits with the physical

develop-ments ofthe country

we

may

gainsomething

by

our studies

with which the strictestutilitariancannot findfault.

Orography.

Mr. R.

M.

Johnstonsupposes that

many

of the

Tasmanian

mountains are dueto the shrinkageoftheearth's crust; but it has long since been proved that the elevations of our mountains are far in excess of that required

by

the cooling

of the world,

and

although

some

slight puckering of the

surface

may

be due to this cause, still I believe itwillbe

generallyconcededthatother agencies are also atwork, and

that the presenceofthe oceanis the greatest of these.

We

almost invariably find that the highest mountain

ranges havebeen developed

more

or less parallel to the past

or presentsea coasts that are

washed by

the largestbodies of

water.

Thus

in South

America

we

find the

Andes

on the

west higherthanthe mountains oftheeast coast; also that

the western shores of

America

are

washed

by the Pacific

Ocean, whichis greaterin area

and

depth thanthe Atlantic, whichisonthe eastern coast.

Coming

nearer

home

for

an

26 NOTES ON THE

MOUNT

LYELL DISTRICT, TASMANIA.

east coast,the Pacific again being larger than the Indian

Ocean. In Tasmania, however, matters are

somewhat

different, for thereis an unbrokenstretch ofwater from its

westcoast to South America; while onits east coast,

some

twentydegrees distant,

New

Zealand acts as a breakwater,

andbraces

up

the ocean, so to speak, relievingtheeast coast of

Tasmania

of

much

pressure fromthe

main

body of water.

Therefore

we

arenot surprisedto findthat the backbone of

Tasmaniais on the western side,especially

when we

find a

map

inWallace's " Island Life " showing thedeep water on

thewest coast at 2,600 fathoms, against 2,000 fathoms on

theeast coast.

We

not only havethe horizontal pressure of

the ocean,but alsotheverticalpressure,andtheforce is not

a steady one, butvaries with thetides, this

movement

being

more

effective in shifting the earth's crust than a dead

pressure.

The

pressureof the

immense

bodies of water in

theoceans are workingas bard to-dayin the building up,

and

indirectly inthe pullingdown, of ourmountainsystems as theyhave inthepast,andit

makes

itspressurefelt

second-hand

by theenormous weight ofthe rocks that they pile up,

whichin theirturnexert avertical pressure.

We

generally

findthat the actualcoastalrange is not so high as the next

inlandone, as if the forcecausing the mountain system

had

limited the area onwhichtherange nearesttoitcould

draw

;

also that theranges following the

main

one are lower

and

further

away

from each other asthey goinland. So in the

case ofTasmania

we

find the

West

Coast

Eange

compara-tively low.

Then

comes the

Main

Range, including the

Frenchman's Cap,

Eldon

Eange, andthe Cradles, which are

among

the highest mountains of Tasmania, These are

followed

up

further inland by the

King

William Eange.

These ranges are like

immense

fossilwaves rollinginland.

They

have alongslope ontheirwestern side

and

a steepone ontheireastern,just asif theyhad been pushed

up by

the

sea.

Having

taken a flying survey ofthe physical features of

Tasmania

we

will

now

make

a few remarks relative to the

mineral wealth of the

West

Coast,

and more

particularly to

the

Mount

Lyelldistrict.

GrEOLOGY.

The

West

Coast of

Tasmania

is rich in thevariety of its

minerals,

among

which ores of the following metals are

found:

—

Tin, gold,silver, lead,zinc, copper, iron,chromium, nickel,bismuth, wolfram, iridium, antimony,manganese,

and

mercury; also other minerals of commercial value

—

e.g.,

marble,serpentine, asbestos,lignite, etc.

Mount

Lyell

(named

after the celebrated geologist, Sir

CharlesLyell)

—

and

itsdistrictis amostinterestingone

from

BY F. D. POWER, F.G.S. 27

different types of ore deposits.

The

most widely known,

perhaps, is thatcalledthe "_{Iron Blow,"on} account of the

hugh

boulders of_{hematite (specimen 556) outcropping on the}

surface.

Mounts

Lyell and

Owen

(named

after Professor

Owen)

are twopeaksofthe

West

CoastRange,dividedbythe

Linda Valley, but connected on their western slopes by a saddle, which forms a watershed, the drainage on the east

flowing

down

the Lindaintothe

King

River, andthat onthe

west finding its

way

into the

Queen

River. That portion of

the Iron

Blow

worked bythe

Mount

Lyell Gold

Mining

Company

issituated on the eastern side of the

Mount Owen

end ofthe saddle.

The

country rock about here consists of

sandstones,

green—

(specimen 546)

and

red (specimen551),

bleached at the surface (specimen 553); various schists

—

ferruginous (specimen 550), talcose (specimen 548),

hydro-mica (specimen 547), etc.; and limestone (specimen 552),

whichhas a schistose structure.

The

general strike of these

rocks is approximatelyparallel with the

West

CoastRange,

aud

also the sea coast in these parts.

The

older rocks are

overlaidwith a quartz conglomerate (specimen549), which

caps the highestpoints of

Mounts

Lyelland Owen.

TheIronBloiv Deposit.

—

Mr. G.

Thureau

believes this lode

to be due tothermalaction, butafter carefullygoing over the

ground threeorfour times I failed to find anyfacts which wouldconfirm thistheory.

Some

people arepleased to considerthisa so-called "true

fissurelode,"butthisfissure theory, which has been applied

to the majority oflodes, is fast dying out, for, assuming a

fissure to be formed for the sake of argument, any

man

accustomedtomining isaware that conditions which

would

enable the walls of thatfissure to keep open until the inter-mediatespacewas filled up with mineralmatter by

means

of

water

must

be very rare indeed. If the country rock has

simplybeen loosenedby

some

dynamicalagency, or even ifa

fault caused a fracture and displacement of the rock, any cavities due to irregularitiesofthe wallswould be filled

up

withrubbedoffportions ofthesideswhich havebeen loosened

by the movement, thus packingthe space betweenthe walls

likemullockin a worked-outlode. Thereis,then,no fissure,

and

thewhole affair

must

be relegated to an exaggerated

form ofloosened country, the interstices ofwhich sve filled

with metallicminerals broughtin by solution, which cement

together the brecciated

and

powdered rock, sometimes even

replacingit. However, the Iron

Blow

shows no signs ofthis,

but, on thecontrary, gives everyindication of belonging to

thatclass of dejjosit

known

assegregation lodes.

The

two prevailing tints thatthe sandstones andschists assume

when

not bleached are green

and

red.

Some

of the

28 NOTES ON THE

MOUNT

LYELL DISTRICT, TASMANIA.

others, atthe expense of their neighbours, and at times

we

get very dark shades indeed; in fact, in places it is

com-pletely turnedintohematite,whichretainsthe

same

laminated

structurethat theoriginalrockpossessed. This

transforma-tioncan be tracedinits various stages until

we come

towhat

we may

consider the adultorperfectstate,as exhibitedinthe

Iron Blow. This deposithas notbeenactuallytracedforany

greatlength, buta similar class of stonehas been foundhere

and

thereonitscourse so farnorth as

Mount

Sedgwick,but

whether these outcrops belong to the same layer of altered schistor to parallel oneswillnotinterfere withany interpre-tation of theirsegretative character.

Although

the Iron

Blow

is composedfor themostpartof hematite, itis not on

accountofits ironthat it is worked, butbecause itis found

tobe the matrix for gold. Besides hematite

we

also find a

fair

amount

of baryta (specimen558), but, curiously enough.,

very little quartz.

The

footwall is red sandstone, with

quartz, and the hanging wall iron pyrites, which

we

will

speak of later on.

The

general strike is nearly north

and

south,thoughlocallybothstrike

and

underlayvary consider-ably, which is just

what

one would expect in a disturbed

countrylike this.

The

hematite

may

be divided into two

classes, accordingtoits hardness,these occurringinalternate

bands, the softerportion looking as if it had beenloosened

by

friction.

We

now

come

to a very important point. There has

evidently beensome great strain abouthere, as indicated

by

cracks in the sandstone andconglomerate filledwith quartz,

and in the limestone, occupied with calcite; also bythe fact

thatthe conglomerate found on the tops of the mountains

only occurinbroken patchesonthesaddle. I

am

inclined to

think that as the peaks of

Mounts

Lyell

and

Owen

were

formed the land piled

up

at those pointswas robbed from

what

are

now

valleys,the dragged out appearance of the

country leaving one underthat impression.

The

greatweight

of these accumulations could not be without effect on the

weakened

neighbourhood,

and when

settling

down

it would have the

same

effectasifone weretocatch hold of opposite

ends ofa pamphlet

and

press

them

towards eachother, if I

may

be allowed to

make

useofahomelyillustration. If this

tookplace aftermostoftheIron

Blow

hematitewasdeposited

it

would

accountfor some of it appearing to be rubbed

up

along thelines of leastresistance, whichare coincidentwith

the planes of bedding and cleavage. It would alsoaccount

for another deposit adjoining the IronBlow, though entirely

distinct

from

it.

On

the hangingwall of the hematite,as before mentioned,is a largedepositofiron pyrites(specimen

555). If

we

examine thestructure of this

we

findthat

when

remark-BY F. D. POWER, F.G.S. 29

ably close, dense texture.

The

boundary between this

pyrites

and

the hematite is very sharp, which is

what we

would

expect if dueto the cause I suggest; for if such a settling

down

ofthe mountains oneach sidetook place after

thehardhematitewas segregated, there beinglittle cohesion

betweenthe hematite and the softerschists, arupture would

most

easilytake placebetween

them

; and the hematite,not

yielding so readily to pressure exerted end on to its grain,

would break

up

into fine particles that could occupy any

loosened portion of its

body

; while thecountry rock, being

more

flexible, would bend ratherthan break, and so remain

porous until infilitrated mineral solutions filled

up

all the

spaces.

The

country rock about the iron pyrites deposit

occurs atvarious angles ofstrike, as ifwrapping round it,

and

thetalcose schist illustratesthe lenticularnature I have tried to describe on a small scale. Thereare no signs of

sudden rupture having taken place here, but everything

points to a gradual motion, which in all probability is

continued at the present day. Associated with the iron

pyrites

we

findgalena; alsocopperpyrites, withits resulting

secondary minerals

—

viz., malachite, azurite, cuprite,

and

native copper.

The

pyrites deposit occupies a depression or

gully running 25 degrees

W.

ofN., the hollow evidently

beingformed

by

the decomposition of the pyrites, a large

portion of it being dissolved out by water in the form of

sulphate of iron, while that which is oxidised into gossan

remainsbehind as a thin capping,until

denuded

by

atmos-pheric agencies. I account for the pyrites being found so

near the surface

by

the fact that the country here is very

moist, a peaty water oozing out of the button grass, the

reducing action of which would retard oxidation. I also

eredit this waterwith"beingthe agent that reduces the silver

and

copperwhich

we

find in theneighbourhood.

The

water

is so darkly coloured that one almost suspects

Neptune

of

havingupsethis billy-can oftea.

The

massof the pyrites is

very poor in gold, but the gossan (specimen 559) or iron capping in places contains a fair amount. This is only

reasonable, foraton ofgossan is greaterin bulk than a ton

of heavier pyrites,

and

since a ton of gossan

must

be the

productofdecomposition of

much

more

than a tonof pyrites,

theformerwill naturally contain the gold concentratedinit,

unlesswashed

away

mechanically,butthevesicularnature of

the gossan tends to prevent that, and the sulphate of iron

formed would prevent it being carried

away

in solution

chemically.

The

decomposedpyrites,

when

placedon litmus

paper

and

moistened, showsa distinct acid reaction, and this stone,

when

passed through the battery,unless well leached

first

and

washedwith alkalinewater, will eat into the plates

30 NOTES OX THE

MOUNT

LYELL DISTRICT, TASMANIA.

The

Linda

Valley Alluvial

—

Most

ofthe creeks flowinginto

theLindaValley arefoundto be gold-bearing, as is also the

alluvial ofthevalley itself; infact, itwas byfollowing

up

the

auriferous debris tothe foot of the Iron

Blow

that this

now

well-known depositwas discovered,and forthisreason it is

generally considered that the gold found in the lowcountry

wasshedfromthe hematite. Ihavecarefullyexamined

some

specimensfoundin thealluvial,which show gold clinging to

its matrix. In some instances,it is true,

we

find the gold

attached to hematite, but

we

also find it sticking out of

quartz. However, specimens of either sort are rare,even at

the head ofthe gullies,the goldbeing, asa rule,clean

and

in

fine grains.

Now,

Ibelievethatmostofthe gold

and

alluvial found intheLindaValley

—

andforthe matter ofthat,inthe

neighbouring creeks also

—

isderived fromthe conglomerate which caps the mountains hereabouts.

My

reasons are these:

—

1. Thereis no earthly reason

why

gold should not occur

in the conglomerate if itwere formed from a

gold-bearing country,and

we

know

the

West

Coastto be

auriferous in several places.

2.

The

alluvialattheheadof the Linda Valley is

water-worn, and composed for themost part of quartz

pebbles. In such a positionthe quartz,if

originat-ing from close by, could not have travelled

sufficientlyfar to be so rounded, besides, there is

not

much

quartz in the neighbourhood,therocks

being mostly schists and sandstones.

3.

The

saddle at the head of the valley connecting

Mounts

Owen

and Lyellhas verylittle conglomerate

leftonit, thoughtraceshere andthere can be seen that could not"possibly have fallen

down

from the

higher mountains. If the conglomerate originally

on the saddle

was

ruptured

and

loosened

by

dynamicalstrains it wouldbe the moreeasily

disin-tegrated and washed

down

into the valley by

sub-aerial agencies than the less disturbed

and

more solid rock on the mountains, which would

accountfor our notfinding

much

conglomerateleft

on the saddle; then, again,the pebblescomposing

the gravel of thealluvial in thevalleyare similar

in size and nature to those found in the

conglomerate.

4. Boulders of conglomerate can be seen in various

stages of decomposition on the sides of the

mountains andinthe alluvial ofthevalley. Ihave

washed

the product from a decomposing boulder

standing isolated onits

way

to a gully below,

and

BY F. D. POWER, F.G.S. 31

enquiries

from

local miners, two of

them

—

Jack

Fehey

andSteve Karlsen

—informed

me

that they

had also obtained gold fromthe conglomerate

wash

at different times.

5. That portionof thealluvialdepositwhich iscomposed

of broken

up

schists is very poor in gold; that

made

up

ofred standstonedrift isbetter, while that consisting of quartz pebbles isthe best, and

we

seldom geta " duffer "dish fromit.

At

Karlsen's

face they averageabout five grains per lode,while

at the Linda Valley claim they estimate fourand

two-eleventh grains per cubicyardofdirt.

The

alluvial at the head of theLinda Valley is evidently notin its original position, as can be seen by thehighangle

at which it is found, for the gravel, sand, clay and

lignite, all of which have different angles of rest,

aretiltedupto sucha degree that proves they could nothave

beendeposited in such a situation.

Some

people shirkthe

necessity of explaining the problem of the Linda Valley

alluvial by saying that the whole mass has been turned

topsy-turvy, but there are no signs ofsuch a generalmixing

up

: the tilting of the deposit from theoriginal angle of

deposition is all

we

requireto

make

the whole affair clear,

and such a

movement

in the earth's crusthas evidently taken

place here, asthe bed rockis thrownoutofitsnormalcourse.

The

richestportions ofthealluvialisfound tobe thetop

and

bottom gravels, and wherever certain small, heavy white pebbles, termedlocally "SailorJacks," occurpayable goldis

sure tobe found.

At

Delany'sface they haveobtainedgood

resultsfrom the former,

and

atKarlsen's face first-rategold

hasbeen

won

from the latter.

Some

persons suggest that

the alluvialissimply conglomerate, which has slipped

down

bodily fromthemountains; but thiscan hardly be thecase,

as thealluvial is quiteloose, while theconglomerate is very

compact; besides,theformerisassociated withlignite which

we

do not find on the heights above, thus proving that the

gravelwas deposited gradually where

now

found,andthat the

conditionsthenprevailing were favourableto a dense

under-growth.

The

presence of occasionalpatches of gravelin the

lignite shows that the original organicmass grew in shallow

placessubjectto floods, and itis onlyin such spots that

we

find gold in the lignite, this goingto prove that the gold

in the alluvial has been deposited mechanically, and not chemically,otherwise

we

would expect tofind itrichernear

the lignite, which would tend to reduce it from solution,

either

by

virtue ofthe organic substanceitself, orelsebythe

sulphurettedhydrogengivenoffby its decomposition. Drift

wood

may

have contributed

somewhat

to these lignite

32 NOTES ON THE

MOUNT

LYELL DISTRICT. TASMANIA.

most

ofthe vegetation grew insitu; wellpreservedsamples'of

King

William pine (specimen 563), and leatherwood _(?)

(specimen 562), can inplaces bepicked out, and sometimes

the ligneous matter has collected iron pyrites about it

(specimen 560).

A

little cement is

now

and again to be

met

with: this inallprobability

was

caused

by

pyritesbeing

formed, an opportunity being given tothe iron present to

obtain sulphur from the decomposingvegetable matter,

and

the

more

heavilychargedwater finding its

way

to the

com-paratively stagnant bottom would cement the pebbles

together. In course of time, as the deposit was drained,

this pyrites would become oxidised, as

we

find it at the

present dayinKarlsen'sface.

Many

of thestreams on the

Mount

Lyell side ofthe gully have been

worked

for gold, until the heavy conglomerate boulders have

become

too

numerous

to remove. Besides gold, native copper is also

obtained.

Mount

LyellCopperDeposits.

—

On

tracingthealluvialcopper toitssource

we

are led toazoneofdecomposedschists,forming

a

pug

_—

grey, yellow,

and

red

—

some

hundred feet broad.

Where

cutby the creeksthis zonewould appeartobe equally

productive throughout, as the tenaciousclay retains most of

thecopper thatis shedon to it; but

when

we

clean'

up

the

bed of the watercourses

we

find the copper is confined to

threedistinct deposits, which runparallel, from twofeetwide downwards.

Where

the

pug

is comparatively dry, a little

below thesurface,itflakes off in piecescorresponding to the

cleavage plains of theshale; while the copper is found in

sheets, as if occupying thejoints and cleavageplains of the

original rock. These sheets of metal, which at times are

fairly thick, get broken

up

into small nuggets, shots,

and

spangles,which are

more

or lesscoated with the black oxide

ofcopper.

Below

the native copper

we come

across cuprite, which occurs in beautifulcrystals, mostlyoctahedrons, with

their edges truncated byfaces of the rhombicdodecahedron.

!No doubtthenative copper hasbeen reduced from thisbythe

agencyofthe peaty waters. Stilldeeperthanthe cuprite

we

come

acrosscopper pyrites, whichtakes

upon

itselfthe

same

form as therockwhich itimpregnates; in fact, it might be termed a cupriferous schist. Instead of copperpyritesthe rock

may

bechargedina similar

manner

withiron pyrites,or

both.

When

the latter takes place the copper pyrites is foundconcentrated on one sideand the iron pyrites on the

other,

and when

the deposit crops outatthe surface the iron

pyrites resolvesitselfintothe oxide,whichissometimes

accom-paniedwithpyrolusite.

Inpresenting the above remarks to the

members

of the

Boyal Society of

Tasmania

Ihavecarefullyweighedthe ideas

BY

F. D. POWER, F.G.S. 33

If the glasses through which I have viewed the works of

naturewill not allow

me

toagreewith the interpretation that

other observershave thoughtfittomake,Icannotbeaccused

of having

drawn

on

my

imaginationto obtrude a pet theory

upon

your notice, for I have endeavoured to give a correct

description ofmatters as they at present are found,

and

to

point out how,

by

using suchforces of nature as are to be

seenat

work

at the presentday, suchresultscould bebrought

about, not forgetting that time is an unlimited factor of

nature's thatenables hertoproduce results slowly,yetsurely,

inthe world's laboratory,which man, duringhis short spaceof

life, canneverhopeto attain bythe

same

means.

DISCUSSION.

Mr.R.

M.

Johnston,F.L.S., in reply toMr. Power,said:

—

Of

the

many

hypothesesadvancedat different timestoaccount primarilyfortheoscillationsofthe earth'ssurfaceitisfrankly

acknowledged bythose

who

espouseoneor other of

them

that

all areopen to objections of

some

kind.

There are three hypotheses, however, ably supported

by

celebrated physicists, which severally

seem

to find greatest

favour

among

geologists,

some

espousing the one

and some

therivalhypotheses. These three hypotheses are mutually exclusive,

and

may

be brieflydescribed as

—

1.

The

contraction or cooling globe theory.

2.

The

gradation or surface displacement theory.

3. Mobility of a hypotheticfluidtheory.

Undoubtedly

the first of these has obtained the greater

acceptance

among

geologists, owing mainly to the able

advocacy ofRobertMallet; and althoughitis admittedthat

there are serious objections toitsacceptance (notably those

urged by

MellardEead

in his "Originof

Mountain Eanges

"),

it is even

now

variouslymodified,the mostwidely accepted

among

geologistsasfurnishing an explanationof the origin

of the upheavals andsubsidences ofthe earth's crust.

Such

isthe opinionofDr. Geikiein his lastgreat work.

The

rival theories, though ably supported, have even

greater objections urged againstthem,

and

even one of the

most

formidable opponents of the contraction theory in

Australasia, ProfessorHutton, cannot urgethatit isdisproved bythe demonstration of the truth of any other hypothesis.

In

his able address before the A.A.A.S. he states that

"

_During

the last fiftyyears investigation has rather been

destructivethan constructive, but progress has been made.

Formidableobstacleshavebeen

removed

. . .

No

doubtthe outlookis stillfoggy,butthe horizonis clearing,

and

we

may

34 DISCUSSION

ON

NOTES

ON

THE

MOUNT

LYELL DISTRICT.

hope that

when

-wehave fuller knowledge of the

movements

ofthe crust

we

shall find a clearexplanationof theircause."

Until that timearrives

we must

be contentprovisionallywith

that hypothesiswhich seemsto accountmostreasonablywith

the greater

number

of

known

facts.

The

proofof a

more

perfecttheory hasnotyetbeen

made

evident.

Me.

Stephens, F.G.S., said that theauthorof the paper underconsideration had evidently paid

much

attentiontothe

geologicalformations of a portionof the "Western Country,

but, in dealing with the question of the history of the

mountain systems

and

the general physical structure of

Tasmania

asa whole,he appeared to have relied too

much

on the information supplied by maps, which is necessarily

very imperfect. Thatthe chief agencyat

work

in producing

those undulationsin the primary rocks which

had

brought

into existence

many

of the mountain ranges and valleys

of

Tasmania

mightbe traced to secular cooling

and

contrac-tion of theearth'scrustthere could be little doubt, andthe

mountain systemsof later date

owed

their origin indirectly

to the

same

cause,

though

thevalleys had, for the

most

part,

been scored out

by

the ordinary processesof natureworking

on

the surface of the country. "In _{addition to}

what

had

been said in opposition to Mr. Power's water pressure

and

wavetheory, itmight be pointed out that until portions of

the crust

had

been raisedthere

was

no deep sea,

and

there-foreallthat the weightof the watercould do, if it

had

any

effect atall,

would

betomoderate the

downward

pressure of neighbouring

mountain

ranges, which are alwaysin a

condi-tion ofcontinuousresistance tothe forceswhich

had

elevated

them.

With

referenceto Mr. Power's remarks on the

con-glomeratesof the

West

Coast, there could beno doubtthat,

as he suggests,

some

of the goldinthealluvial deposits has

been derived

from

conglomerate formed from the waste

of auriferous rocks.

On

theother

hand

it has to be borne

in

mind

that, while there are

many

conglomerates

inter-stratified withthe primary rocks, andlikely tocontain gold

in cases inwhich thecementedmaterials have been derived

from

auriferous veinstones,there are other conglomeratesof

more

recent date which have been derived from bands of

quartz rock,quartzite, andassociated schists, suchas occur

in greatforce at

Mount

Arrowsmith, Rocky Cape,

and

else-where,in which nogold occursto any appreciableextent.

Mr. A.

Montgomery,

M.A.,

Government

Geologist of

Tasmania, regretted the absence of Mr. Power, as he would have liked tohave

had from him

further explanation of the

new

theories advanced.

The

subjectmatterofthepaper

had

avery wide scope, comprisingtwo

new

theories, one of the

formation of mountain chains,

and

a secondofthe origin of

DISCUSSION ON NOTES ON THE

MOUNT

LYELL DISTRICT. 35

Each

of these subjects wasquite large

enough

to require a

separatepaperforits elucidation. Taking the

new

theory

ofmountainelevationfirst, the writer referredto Mr. John-ston's supposition that

many

of the

Tasmanian

mountains

are dueto theshrinkage of theearth's crust,

and

dismissed

thewholecontraction theorywith the cursory

remark

that "ithas long sincebeen proved" that that explanation

was

insufficient.

He

thoughtthat

many

other geologists

would

agree with

him

(Mr.

Montgomery)

indenying thatany such

thing

had

long since beenproved.

While

it might be

con-cededthatotheragencieswereatwork, he did not thinkeither

that

many

would

agree that the

mere

presence of the ocean

was

the greatest.

The

subject of the rate ofthe cooling ofthe

earth

and

the contraction consequent thereon

was

a

most

abstruseone,

and

there

had

been great difference of opinion

expressed aboutit. In his mentionofthe higher

mountain

ranges, the writer left out of account the great range of

mountainsextending

from

the westof

Europe

to theeast of

Asia,

and

right

away from

any large

body

of water.

The

theoryenunciatedwasthatthe

mere

weightof

huge

bodies of

waterpressingonthecrust ofthe earth forced

up

a partofthe

solid crust toahigher level,

and

he failed to see

how

such a

thing couldhappen.

Water

could only raise watertoits

own

level,

and

could not raise a heavier substance to a higher

level.

The

theory brought forward to account for

Mounts

Lyell

and

Owen

wasa curiousone,

and

he could not see the

probability ofit, unless indeed the change

was

ascribed to

some

grabbing influence

from

outside the earth altogether,

that being the only sort of force that could rake

up

the

material

from

thevalleys

and

give the country a "dragged outappearance." Thisand the further referencetotheeffect

of " the greatweightof these accumulations" which are no

soonerraised

up

thanthey beginto settle

down

again,

pre-sumablydirectlyagainst the forcethathasjust raised them,

led one to think that the writer

must

have based his theory

onthesupposition that thewholecrust ofthe earth

was

ajelly,

and

he suggested thatthis theory

might

be

known

astheJelly

Theory, to contrast it with those generally accepted.

The

"homely

illustration" ofthepamphlet wasnot a good one,

astheforces thereappliedwere twolateral ones in opposite

directions,while thatto be illustrated by it was one force

actingvertically.

The

writerstatedthat the " fissure theory, which has been appliedto the majorityof lodes, is fast dying

out;" but hisimpression

was

that 999 out of every 1000

geologists or

men

connected with mining

would

hold that theoryasonethat

met

the greater

number

ofcases, andassert

that truelodeswere almost universally found to be fissure

deposits. It

was

afactthat suchpackedspacesas alludedto

36 DISCUSSION

ON

NOTES ON THE

MOUNT

LYELL DISTRICT.

fissures were

common

in lodes.

The

difficultiesof imagining

that a lodefissure could be kept

more

or less open

disap-peared

when

itwas

remembered

that fissures were generally

uneven, and that " faulting," even to a very slight extent, would result ina series of

more

or less connected but

irre-gular cavities. That

many

lodeswere linesof fault was well

known.

He

instanced the case of the

Tasmania

reef at

Beaconsfield, which crosses highly inclined

and

distinctly

bedded

strataalmost at right angles. In the drive on the lodewestwardat No. 6 levela

band

ofblack rockis found

on

bothsides of the drive for

some

distance

from

the shaft.

On

one side of the lode white sandstone then

makes

its

appearance,butthe black stillcontinues on the other side,

and it is not till a further distance of 104 feet has been

passedover that the white sandstone comes in on it, thus

proving a throw of the beds of the country rock of that

amount.

Mr. E.

M. Johnston

expressed general approval with the

criticism ofMr.

Montgomery, and

expressed regret that

Mr.

Power had

not confined^his attention to one of the

many

important subjects introduced

and

treated it in a

more

scientificmanner.

Mr.

W.

F.

Ward,

A.R.S.M.,

Government

Analyst of

Tasmania, said:

—

Mr. Power, _{in his} notes

on

the

Mount

Lyell district, takes exception to the supposition that

"

_many

_ofthe

_Tasmanian

mountainsareduetothe shrinkage

of the earth's crust," althoughhe admits that "

some

slight puckeringof the surface

may

bedue to this cause." In so doing hejoins issue with the great majority of geologists;

for, asGeikie, in summarising the causes of upheaval

and

depression, says:

—

"

With

modifications,the

main

cause of terrestrial

movements

is still soughtin secularcooling " (and consequent contraction). Confining our attention to the

Tasmanian

mountains,

we may

say that the highestpeaksare

one mileinheight, the diameter of the earth 8,000 miles ;

the whole of this cannot therefore be called

more

than a "slightpuckering,"

and

itwould ona2ft.globeberepresented

by

the thickness of thinforeignnote paper.

As

regards the

allegedinadequacy of the shrinkage to produce this

com-paratively slighteffect, Mallet, the great authority on this

branch of the subject, estimates that the diameter of the

earth

when

liquid

was

at least 189 miles

more

than it is

now;

this implies alsoa shrinkage ofthe circumferencetothe extentofnearly600miles, sothat

we

seemtohavea veryample

margin, sufficienttoaccount not only for Tasmania's

com-parativelysmall hills,butforsuch enormousrock masses as

theHimalayas

and

theAndes.

Not

satisfiedapparentlywith

this,however, Mr.

Power

invokes other agenciesfor the

for-mationof mountains,

and

says:

DISCUSSION

OX

NOTES ON THE

MOUNT

LYELL DISTRICT. 37

1. "

The

presence ofthe ocean isthe greatest of these."

2. "

We

havenot only the horizontal pressureofthe ocean,

butalso theverticalpressure,

and

the force varies with the

tides, this

movement

being

more

effective in shifting the

earth's crustthanadirectpressure."

3. "These ranges are like

immense

fossil waves rolling

inland.

They

have a long slopeontheir western side, and a

steep one on their eastern, just as if they

had

been pushed

up by

thesea."

The

nearest approachto Mr. Power's suggestions which I

have beenable to find,isthatof

some American

geologists,

viz.:

—

"

That

the removal of rock by denudation from _one area,

and

its accumulationin another affects the equilibrium

of theearth's crust,

and

causes subsidence where deposition

takesplace, while the

denuded

areabeingrelieved of weight

rises;" this, however, has been fully

met

by

proof that

"shouldtheremoval and depositof a few thousand feet of

rock so seriously affect the equilibrium of the crust as to

causeit torise

and

sink inproportion, it would evince such

a mobilityinthe earth as could notfailtomanifest itselfin a

far

more

powerful

way

undertheinfluence oflunar and solar attraction." If,then,thepressureof afew thousandfeet of

rockbeincapable of raising the level of neighbouring

dis-tricts toany appreciableextent, stilllesscan the pressureof

water, which has only aboutone-third the specific gravityof

rock,becapableof slowlyraising land,

and

sending itrolling

inland"like

immense

fossil waves." If suchwere the case

stillgreaterwould be the mobility

and

theinfluence of lunar

and

solar attraction.

With

regard to the Iron

Blow

I have found

some

difficultyinfollowing Mr. Power'stheory; butin view of thecloseneighbourhoodof the iron pyrites and the hematite, ofthefact thateachis schistose in structure, that

each contains

much

barytes,

and

that thehematite is on the

footwall of the vein or deposit, I

am

still of the opinion

expressed sometime back in reply toMr.

Thureau

: that the

Iron

Blow

itselfis the result ofthedecompositionof

some

of

thislarge deposit of iron pyrites. I will only allude to the

question of "fissure" veins to point out that Mr.

Power

quotes no authority against

and

suggests no substitute for

"this fissure theory," as he termsit. In conclusion I can

only agree with Mr.

Montgomery

that Mr. Power,

notwith-standinghis disclaimer, has

drawn

onhis imaginationfor his