486 WHITMAN CROSS 



same texture and composition demonstrably derived from sedi- 

 ments or occurring intercalated in the sedimentary series but not 

 clearly of igneous origin. Excluded from the group under dis- 

 cussion are the primarily banded igneous rocks and the metamor- 

 phic derivatives of igneous rocks whenever that origin can be 

 established, and whatever the process of change may have been. 

 In other words the group includes the rocks below the oldest 

 known sediments so far as they are not visibly eruptive or 

 igneous and all later rocks of the same characters derived from 

 sediments or of unknown origin. 



This group then has nothingin texture or composition to distin- 

 guish it. Neither of the elements of the name has any restrictive 

 significance. The group is geologically homogeneous only in 

 case the schists of unknown origin are actually derived from 

 sediments. If, as many suppose, a large proportion of the 

 Archean gneisses, etc., represent igneous masses, metamor- 

 phosed or not, the group is not only heterogeneous from the 

 genetic standpoint but causes the separation of identical things. 



In the subdivision of the crystalline schists mineral composi- 

 tion is applied, the predominant constituent causing the reference 

 of a rock to a certain group. The terms gneiss and schist are 

 not defined. 



The group of crystalline, or non-clastic sedimentary rocks, is 

 heterogeneous in constitution as is apparent from a partial list of 

 the rocks referred to it : ice, cryolite, limestone, opal, quartzite, 

 porphyroid, iron ores, coals, diatomaceous earth. That such a 

 group lacks the unity required in a systematic division, and that 

 its descriptive name by no means covers the case, is apparent at 

 once. It is confessedly a grouping for convenience only, and 

 embraces, in fact, the remaining rocks after the other three have 

 been established. 



H. Rosenbusch, i8g8. — A comprehensive discussion of rocks 

 was issued in 1898, by H. Rosenbusch, entitled, Die Elemente der 

 Gesteinslehre. 1 Although much less detailed than the Lehrbuch of 

 Zirkel, this work is of much interest as expressing the views of 



1 Stuttgart, 1898, pp. 546 + 4. 



