Reviews 



Structural Geology. By C. K. Leith. New York: Henry Holt & 

 Co., 1913. Pp. 169; figs. 68. $1.50. 



The central feature of this admirable work is the interpretation of 

 rock structures as expressions of dynamic processes, rather than descrip- 

 tions simply. Students of geology are generally more or less familiar 

 with the different kinds of rock structure, simply as such, but they 

 are not so generally accustomed to interpret these structures as related 

 parts of a record or of a process. In this work the author has adopted 

 a philosophical mode of treatment by approaching rock structures from 

 the point of view of the forces and processes which have produced them. 

 The structural phenomena are aligned chiefly as the products of fracture 

 or of flow. 



As the book is relatively short, the author plunges at once into a 

 critical discussion of rock fracture and rock flowage and of the con- 

 trolling conditions which express themselves in one or the other of these 

 two methods of deformation. Then in the light of the principles of 

 rock fracture which have been developed, he takes up in detail the treat- 

 ment of the leading rock structures resulting from this type of deforma- 

 tion. Of these, faults naturally receive the greatest attention, but a 

 fair apportionment is given to joints, fracture cleavage, breccias, and 

 autoclastics. Earthquakes, both as causes and effects of rock fracture, 

 complete the first half of the book. 



In the second half, under the heading of rock flowage, are treated 

 in order flow cleavage, gneissic structure, and porphyritic textures 

 developed by rock flowage. These lead to a digression on the identi- 

 fication of schists and gneisses, in which the stand is taken that with 

 our present knowledge, field observations are likely to yield more satis- 

 factory conclusions as to the igneous or sedimentary origin of given 

 gneisses and schists than either mineral or chemical composition. Folds 

 are treated as structures common to both the zone of fracture and the 

 zone of flow, but folds developed in these two zones may be discriminated 

 by the contrasts which they exhibit in a number of particulars. 



Mountains and the other major units of structure each receive 

 a brief but pointed consideration. Isostasy as an agency to explain 

 these relief features is analyzed and then criticized on a number of 



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