268 Revieics — Dr. M. E. Wadsworth — LitJwIogical Studies. 



is a fii'st step towai'ds a cleavage structure, which is rather different 

 in character from that described at the beginning of the present 

 article, though similar in its effects. This has been studied by 

 Dr. Sorby in the slates of Liskeard and other places, and is 

 described and figured by Heim under the name Ausiveichungsdivage. 

 I suppose Mr. Fisher refers to something of the same kind when he 

 speaks of a frilled schist passing into a rock showing schistose 

 cleavage (p. 176). 



In conclusion, I should like to correct a misapprehension of Mr. 

 Fisher's with reference to my former paper. I have always supposed 

 the direction of cleavage to indicate the plane perpendicular to the 

 maximum compression (i.e. the principal diametral plane of the 

 ellipsoid of distortion), and not the direction of shearing (which 

 would be, on Mr. Fisher's hypothesis, one of the circular sections). 

 For a great amount of shearing, however, the two planes would 

 make only a small angle with one another. 



My diagram, like that of Mr, Fisher to which it corresponded, 

 was not intended to have any relation to the surface contour of the 

 land. 



S, IE V Z IE AAT S. 



I. LiTHOLOGICAL STUDIES, A DESCRIPTION AND CLASSIFICATION 



OF THE ROCKS OF THE CORDILLERAS. By M. E. WaDSWORTH. 



(Memoirs of the Museum of Comparative Zoology at Harvard 

 College, vol. xi. pt. 1, pp. 208 and xxxiii. plates viii.) Cambridge, 

 Mass., Oct., 1884. 



THE science of petrology, or petrography,^ as Dr. Wadsworth 

 would prefer to call it, has been cumbered, more perhaps than 

 any other, by crude hypotheses and wide generalizations founded on 

 slight bases of facts. To students weary with this kind of literature 

 Dr. Wadsworth's Lithological Studies will be a welcome refreshment. 

 The method of treatment is logical, which must command respect 

 even if it failed to convince ; the style is clear, and not seldom 

 incisive. Dr. Wadsworth in controversy " calls a spade a spade," 

 and bursts the bladders of tumid hypothesis with scant ceremony. 



The present volume is but an instalment of a work which must 

 extend to a considerable length. Its basis, as implied by the title- 

 page, is the lithological collection accumulated by Dr. Whitney in 

 the process of the (uncompleted) Survey of California, but in dealing 

 with the more basic portion of these rocks in the present part Dr. Wads- 

 worth has found it needful to carry his investigations far beyond the 

 limits of the Coi'dilleras. The opening chapter treats briefly of the 

 structure of the earth. The author thinks it probable that the inmost 

 portion concerning which we have any data is composed of iron, with 



* On the analogy of the names of all the other sciences there can, we think, be no 

 question that this use of the term petrography, whatever may be the authority in its 

 favour, is wrong. Dr. "Wadsworth uses petrography as inclusive of lithology and 

 petrology, the latter dealing with the characters of rocks observed in the field only : 

 but surely this is merely petrography ; the science is petrology. 



