SYSTEMATIC PETROGRAPHY 473 



their origin, he thinks that every system must so largely repre- 

 sent individual opinion that probably no one system can ever 

 receive universal recognition. 



The systematic arrangement of Roth is, in its general out- 

 line, as follows : 



A. Rocks composed essentially of minerals. 



I. Plutonic (consolidation products of molten magmas). Free from 

 fossils, composed of minerals or substance chemically like a mineral 

 aggregate. 



1. Eruptive. (Breaking through other rocks.) 



a. Pre-Tertiary. 



b. Post-Cretaceous. 



Appendices to a and b contain rocks produced by weathering. 

 Tuffs. 



2. Crystalline schists. 

 Appendix-Weathering products. 



II. Neptunic. 



1. Partly fossiliferous ; composed of minerals and of the products of 

 the decay, decomposition, and attrition of minerals. 



a. Precipitates from solution. 



b. Deposits from suspension. 



2. Clastic, composed of rock fragments. 



R. Rocks composed essentially of organic remains. 

 C. Products of contact metamorphism. 



It will be observed that the ancient crystalline schists are 

 regarded as the primary crust of consolidation of the earth. 



The geological factors of origin, relations, or age, are variously 

 applied in the construction of this scheme, and in constitution 

 the distinction between mineral and rock particles is made. All 

 Plutonic rocks are regarded as consisting essentially of silica or 

 silicates, excepting that in the crystalline schists carbonates appear. 

 The silica free minerals — apatite, magnetite, ilmenite, etc., are 

 treated as accessory constituents. This exclusion of the latter 

 group of minerals from a position of systematic importance is 

 not discussed by Roth, but its evident result is that in certain 

 rocks, e. g., those rich in magnetite, the components do not have 

 their natural and logical weight in classification. 



