474 WHITMAN CROSS 



Of the pre-Tertiary eruptive rocks, Roth makes, for con- 

 venience, three divisions : 



I. Orthoclase rocks. 

 II. Plagioclase rocks. 

 III. Peridotites. 



The first two of these groups should, logically, have been 

 united systematically in the division of Feldspathic rocks, 

 including all with appreciable content in feldspar, since the 

 Peridotites are defined as free, or nearly free, from feldspar. 

 The question of recognizing the quantitative relations of mineral 

 constituents is not mentioned by Roth. 



The silicate minerals are applied by Roth for the subdivision 

 of the three main groups in the usual way, and by means of 

 structure the granular, porphyritic, and glassy varieties are 

 distinguished. 



In the detailed treatment of Eruptive rocks, as in the arrange- 

 ment of Crystalline schists, the Neptunic rocks and the Classes 

 B and C, mentioned above, Roth's order of presentation and 

 discussion can hardly be said to be systematic. It is an arrange- 

 ment for convenience of description, not based upon the logical 

 application of principles ; and it is, therefore, not desirable to 

 devote more space to its analysis in this review. 



E. Kalkozvsky, 1886. — A condensed text-book on rocks was 

 published by E. Kalkowsky, in 1886, with the title Elemente der 

 LitJwlogie. x For the primary division of rocks the author formu- 

 lates an original criterion, and proposes two great classes : 



I. "Anogene" — -of which the material came to the place of rock forma- 

 tion from below. 



II. " Katogene " — of which the material was derived from above. 



These correspond closely to the eruptive and sedimentary 

 divisions of other authors. 



For the classification of the "Anogene" rocks Kalkowsky 

 applies the following factors: (1) Chemical composition as 

 represented in mineral composition; (2) the usual age distinc- 

 tion ; (3) structure. He rejects genetic distinctions as unsuit- 



1 Heidelberg, 1886, pp. 316. 



