Review

REVIEW

PALAEOGEOGRAPHrSCHER ATLAS DER UNTERKREJDE VON NORDWESTDEUTSCH-LAND. By Wolfgang Schottet ai,1969. Budesanstalt fuer Bodenforschung, Hannover, West Germany.

This publication is a colossus of maps, text figures, and explanations dealing with the Lower Cretaceous of Northwest Germany in great detail and presenting an 'over-view' of the period as recorded in the rocks of the whole of northern and central Europe. A stratigraphic chart of the Lower Cretaceous succession as devoloped in northern-central Europe (extending from Greenland in the west to the central Russian platform in the east) is also included.

The work is a result of scanning and synthesising the description and data found in 1114 references dealing with northwestern Germany, 246 references concerning western part of north-central Europe, 29 references covering the northern part of north-central Europe and 323 references dealing with the eastern part of north-central Europe. It is also a result of the study of geological maps covering northwest Germany (77), western part of north-central Europe (31), Sweden and Denmark (2) and eastern part of north-central Europe (18). These figures are mentioned to indi-cate the' quantum' of thorough work done in producing the work under review.

The work is in 3 volumes one of which forms the text and the other two being maps. The text (explanation) consists of 315 pages of which 18 form the text figures. Chapters 2-5 cover the material studied and the methods followed in preparing the maps and the stratigraphic details ranging in age from Wealden to Albian. In chapter 2 is given a discussion on the methods followed in the preparation of paleogeographi-cal and paleogeologipaleogeographi-cal maps. There are also as many as 67 paleogeological maps included in the work under review. Chapter 3 deals with the stratigraphy of the whole region of so-called north-central Europe. Chapter 4 deals with the explana-tion of the maps covering the area of northwest Germany only, about which there are 299 maps, 69 of them being paleogeological maps. Chapter 5 explains 5 paleogeo-graphical and 2 paleo geological maps of north-central Europe. Chapter 6 provides summaries in French, English and Russian languages, very brief summaries to indicate the scope of the work and the results of the synthesis attempted in the work.

The publication under review is a 'trend-setter' in the field of paleogeographical and paleogeological studies. Itis a veritable store house of information about the development of sedimentation during the Lower Cretaceous in north-central Europe, particularly northwest Germany. Students of Lower Cretaceous stratigraphic geo-logy will find in this work a regional portrait of events that occurred both on land and in sea over the north-central European region, especially in those six maps on a scale I : 1,500,000 and in the stratigraphic Table; and therefore they owe a lot to Prof. Schott and his colloborators and to Bundesanstalt fuer Bodenforschung for bringing out this valuable publications.

PERTIES OF ROCKS-By W. Dreyer. Trans. Tech. Publications U. S. A. 1972 Price 25 dollars.

This publication deals with and comprises primarily the relationship between state of stress, strength of rocks and their characteristical textural data. Prof. Dreyer in this monograph deals with a very important aspect concerning the mechanical behaviour of rocks giving due consideration to the petrographic parameters such as mineral composition, mineral interlocking, grain density and porosity. This method of analysis, drawing heavily on several important branches of basic sciences viz., physics and geology bordering on geophysics, is the novel feature of this monograph. Special emphasis has been laid in this monograph on understanding the geo-logical and tectonical phenomena for the design and construction of civil engineering edifices. Studies in the field of experimental petrography, behaviour of rocks under biaxial and triaxial conditions and determination of absolute rock pressure at a specific point in a mine have been extensively used for the understanding of mechanical and tectonic behaviour of all kinds of rocks.

The monograph of 500 pages has been divided into three sections and 18 chapters. Very valuable data on the elastic constants of all kinds of rocks under different condi-tions of pressure have been detailed in this book. The behaviour of rocks under diffe-rent experimental conditions has been correlated with geologic features both in the field and in the laboratory. Several interesting problems in the fields of rock mecha-nics, civil engineering, mining engineering and basic physics and geology have been discussed. The monograph gives a comprehensive account on the strength properties of rocks.

Prof. Dreyer, a distinguished worker in the field of rock mechanics, has done a yeoman service in compiling such excelIent data in the form of a monograph which will be invaluable to the worker who is keen to understand and solve his rock mechanics problems. As the author has himself stated, it is hoped that this volume will lead to the better understanding of the mechanical and tectonical behaviour of all kinds of rocks not only for the mineralogist, geologist, petrographer and mineral deposit scientist, but will also give the civil, geological and especially the mining engineer the important basis for solving his rock mechanics problems. More than this, I envisage that this monograph will open up new vistas into the realms of rock mechanics and give a good insight to the young worker.

S. DALAKRISHNA

MORPHOLOGIC CRYSTALLOGRAPHY AND OPTIC MINERALOGY: By S. Ray. Oxford& IBH Publishing Co., Calcutta, 1971, XIV+290 pp., 101 Figs., Price: Rs.

18/-The long felt need for a good text book on morphologic crystallography and optical mineralogy by an Indian author for use by post-graduate students in geology ofIndian Universities is at last fulfilled with the publication of the book under review. The author of the book-Professor S. Ray-is very well known as a teacher and re-searcher, who has produced several equally brilliant students. As the editor of the Quarterly Journal of the Geological, Mining and Metallurgical Society of India for over two decades, he was closely connected with scientific and research writings in India and abroad.

REVIEW 433

thirty years in two Indian Universities and is written in such a manner that the princi-ples discussed and the procedures taught in the classes are recoverable easily, clearly and fully from the book. The book is profusely illustrated with freshly designed three-dimensional drawings, which can be easily reproduced by the student. The author has shown originality while presenting certain concepts and procedures.

In spite of extreme care taken in the preparation and printing, the book can be criticised from several points of view. The prsentation of material under' Optical mineralogy' is quite satisfactory, but the same can not be said for 'Morphologic crystallography'. In the name of originality some deviations have been made in presenting the material in this part. Instead of using the widely known Hermann-Manguin symbols for describing the symmetry of point groups, a different notation having no special advantage is used. The distinction of polar and diagonal planes by using symbols like 2m and 2-m is confusing and does not serve any useful purpose. The method of deriving crystal classes is novel, but is in no way superior to the ones described in standard books. It is irrelevent to use terms like 'equatorial' and , polar' for describing the symmetry elements of isometric crystals, where all crystallo-graphic axes are interchangeable.

The nomenclature used to describe crystal classes is again novel, but is in no way superior to any of the existing ones. There is no special advantage in adding one more type of nomenclature, when some of the existing types of nomenclature like the one based on the nature of general forms (hexoctahedral class, etc.) and the other based on symmetry (tesseral holoaxial class, etc.) are quite satisfactory. In the present nomenclature, it is not always possible to identify a crystal class without knowing the crystal system.

The use of the term, ' hemimorphic axis' to describe a 'uniterminal axis' is con-fusing. This gives an erroneous impression that all crystals having hemimorphic axes are hemimorphic classes and one wonders why crystals belonging to 43m, 23, 6m2,

6,

The usage of the modern nomenclature of crystal forms where the name of a form is not changed by change of orientation is commendable. A few mistakes, however, have crept in the description of crystal forms. Itis wrong to say that in 2m class {hoI} and [okl} are prisms (p. 103). They should be domes (or sphenoids ?). In the same way, it is wrong to describe {hoi} form as B-prism in monoclinic system (p. 106). This form should be B-pinacoid. Itis unfortunate that the author is very critical in finding it unjustified to recognise dome and sphenoid as independent forms; but most of the modern crystallographers find it justified to make this distinction, and this distinction is actually necessary if one wants to describe crystal classes by their general forms.

Although many diagrams are useful and reproducible by students without any difficulty, some mistakes are noted in them. Figures 79 and 91 are to be interchanged. In some cases, the orientation of crystals is wrongly shown (Fig. 55). Some figures (Fig. 105) are too small to be of any use, while some other figures (right hand side of Fig. 5 and Fig. 106) do not convey the required details. In some stereograms, planes which are not symmetry planes, are erroneously represented as solid lines. There is no special advantage of repeating stereograms of each crystal class by giving rotary axes in one and inversion axes in the other. It is confusing to use a special symbol for inversion diad axis. In Fig. 104, the symbol for inversion diad axis should be at

In the optical mineralogy part, the usefulness of the book could have been enhanced by introducing the use of the Universal stage in the determination of optical properties and the use of Berek Compensator in the determination of birefringence. In spite of the above, the book has so many merits that it can be recommended as a text book for post-graduate students in geology in all Indian universities.

M. G. CHAKRAPANI NAIDU

STRAIN FACIES by Edward Hansen. George Allen & Unwin Ltd., London, and Springer-Verlag, Berlin-Heidelberg-New York, 1971. X

+

This is the second monograph in the series' Theoretical and experi mental studies of minerals, rocks and inorganic material'. According to the prefatory remarks of the general editors, the purpose of the monograph series is to publish reviews on recently developed scientific methods and their results,

The author begins with the thesis that the facies concept (' the present character of a rock expresses its conditions of formation ') is useful not only in deposition and recrystallization phenomena in rocks but is also a powerful tool in analyzing the deformational features of rocks. Structural features characteristic of a particular strain facies must have developed under a particular set of strain environments. The structural features that have been utilized by Hansen for classification into different facies are folds in hand specimen and outcrop-the mesoscopic folds. The examples are taken from the middle-grade metamorphic rocks in Trollheimen, Norway. The folds are described by twelve properties, eight of which are characteristic of individual folds; four other characters are seen in a group of folds. These properties are (1) geometry (parallel or similar), (2) nature of hinge and limb (broad, sharp, straight. curved), (3) ratio of short limb height to width, (4) ratio of depth to width, (5) length and character of hinge-line (short, long, straight, curved), (6) cylindricity (cylindrical. cylindroidal, irregular, conical), (7) relation to cleavage, (8) relation to mineral lineation: properties of a group of folds used are (9) mean and standard deviation of ratios of short-limb height to width, (10) mean ratios of depth to width, (II) preferred orientation of fold axis; and (12) asymmetry. A combination of all these characters lend to the folds their' style', and, on the basis of style, three groups have been identified in Trollheimen region.

REVIEW 435

facies. These three chapters (pp. 64-173) form the bulk of the book. Here the author describes the twelve properties of the folds in each facies. In the final chapter the general concept of strain facies is discussed in the background of the structures of the Trollheimen region. A summary of characters of the three strain facies and a

strain-facies map of the area are presented. A fairly extensive list of references (up to 1969) and an index are appended.

The book will surely be useful as a description and interpretation of complex structures from an area. Some of the photographs, particularly of interference struc-tures and superposed folds, are excellent. The simple method of deducing slip-lines in superposed folds (provided the later folding is by slip) may be useful. Itis however doubtful how mineral lineation and cleavage will help in classifying folds. Also, cylindricity is bound to decrease after the folding of the first phase in any area of superposed deformations. But more important than these, a large number of infer-ences by the author are open to serious criticism. The identification of axial-plane cleavage in two of the three facies as slip-cleavage will not be accepted by many struc-tural geologists. Also, the identification of slip-folding on the basis of' similar

geo-metry' and the distribution of early lineations in a plane is controvertible. These

conclusions would have been valid if the author could establish independently that the later folding is not by buckling (on the basis of thickness/length of arc measure-ments). Itis surprising that in a work primarily concerned with folds the author has not mentioned the studies of Biot and of Ramberg anywhere, nor has he included their publications in the list of references. As has been pointed out by a number of workers in recent years, 'similar' geometry of a fold does not automatically indicate slip-folding. An almost similar fold can be generated by buckling followed by flattening and flow. The early lineations will fall in a plane (great circle in stereographic pro-jection) so long as the later folds have a 'similar' geometry. Hence, a fold formed by flexure followed by flattening can also distribute the early lineations almost in a plane. The author's contention of 'the geometrical impossibility of forming tight interference structures by late flexural-slip folding without creating voids between the layers or without developing subordinate fords and faults' (p. 32) is also debatable. Whether an interference pattern of dome-and-basin type will show subordinate struc-tures or not depends on the scale of observations. The reviewer can cite many instan-ces of interference patterns (of the type described by the author) formed by late flexural-slip folding (proved by the small-circle distribution of the early lineations, in addition to the criteria of buckle folding), which may show' simple' geometry locally. Lastly, as has been increasingly appreciated in recent years, the style of a fold-the overall pattern of a fold emphasized by Hansen-may vary in folds of the same generation and in the same rock types, just as folds of different generations and apparently belonging to different facies may show the same style. Laboratory experi-ments with rocks, and foliated and unfoliated model meterials have shown that round-hinged, sharp-hinged and conjugate folds having different styles may develop by the same deformation in different parts of a unit in different stages. There are thus sufficient grounds to question the validity of the three strain facies suggested for the Trollheimen area.

In summary, this publication will be useful to all researchers working in terrains showing multiple folding. But it is doubtful whether this book will be used by many structural geologists as a monograph on general principles and methods of analysis of