476 WHITMAN CROSS 



E. Rocks not included in any of the preceding groups. 



F. Vitreous rocks. 



G. Fragmental volcanic rocks. 



It will be seen that this grouping is mainly mineralogical and 

 does not express the quantitative element in any logical way. 

 It practically recognizes the entrance of feldspar, in any amount 

 above that of the undefined "accessory" role, as creating a 

 large group of feldspathic rocks. The subdivision of these groups 

 is first on some further mineral distinction and after that occur- 

 rence and texture enter combined into the system by distin- 

 guishing rocks of granitic from those of trachytic texture, using 

 these terms in the sense of Fouque and Michel-Levy, and con- 

 ceiving that the result is practically to separate plutonic or deep- 

 seated from volcanic or effusive rocks. Age is not introduced as 

 a factor. 



Within the last twenty years several attempts have been 

 made to apply, in extenso, the chemical composition of rocks for 

 their classification. These attempts have been prompted by 

 various motives. Some appear to have no really practical 

 object, viewed from the petrographer's standpoint ; others are 

 connected with hypotheses of magmatic differentiation ; and still 

 others have been inspired by a realization of the complexity of 

 the problem of a rational arrangement of rocks on the basis of 

 their numerous and highly variable mineral constituents. It 

 appears to the writer, however, that it may be fairly said of all 

 these attempts that they are either not classifications of rocks, or 

 that they are not actually chemical classifications. 



Fra?iz Schr'dckenstein, 1886, i8gy. — Two peculiar attempts by 

 Franz Schrockenstein, an Austrian writer, to discuss the chemi- 

 cal composition of silicate rocks, irrespective of their origin, and 

 upon that basis to classify them, are mentioned here only for 

 the sake of completeness, as these attempts have no direct bear- 

 ing upon a logical system of rocks. The writer has seen only 

 the more recent of the essays in question, 1 depending for the 



*" Silicat-Gesteine und Meteorite," Petrographisch-chemische Studie auf Grund- 

 lage des neuesten Standes der Wissenschaft bearbeitet. Prag, 1897. 



