784 A TREATISE ON METAMORPHISM. 



schistose, or gneissose. If we wish to designate the minerals of which the 

 rocks are composed, we may prefix the names of the minerals to any of 

 the previous names, as, for instance, mica-hornblende-schist-syenite, or 

 mica-hornblende-syenite-schist. Still further accuracy is obtained by pre- 

 fixing a geographical name, as Aurora granite. A final stage of accuracy is 

 obtained by combining all of these devices, as Aurora mica-quartz-feldspar- 

 schist-granite, or Aurora mica-hornblende-syenite-gneiss. Of course ordi- 

 narily the rock will be designated by the simpler terms, but somewhere in 

 a paper all of the qualifiers can be put together, so as to give in a single 

 compound name its most accurate designation. 



The elasticity of this plan, by which it is possible to make loose, 

 approximate, and very fine discriminations, is noteworthy. In the field it 

 may not be possible to give the mineral composition or the origin of the 

 rocks and an expression on these points can be avoided. But so far as the 

 facts are known they may be expressed in the notes if desired. Thus all 

 grades of knowledge of the metamorphic rocks can be indicated without 

 implying more than is known. When additional knowledge is gained and 

 further refinement is possible, these refinements may be expressed. 



In preparing reports in the office it is very often advantageous to be 

 able to throw large groups of rocks together which have common structures 

 or textures without reference to their mineral composition or origin. In 

 another part of the text it may be desirable to indicate the mineral character 

 of the different rocks. In other parts of the text it may be desirable to 

 indicate exactly the origin of the rocks. All these wants are very nicely 

 met by the proposed usage of the terms meta, apo, slate, schist, and gneiss. 



SEDIMENTARY ROCKS. 



The sedimentary rocks may be divided in accordance with the ordinary 

 classification into nonfragmental or nonclastic and fragmental or clastic. 



The nonfragmental rocks may be divided into five orders — nitrates, 

 sulphates, chlorides, carbonates, and oxides. The only important nitrate 

 is niter. The important sulphates are gypsum and anhydrite. The only 

 important chloride is rock salt, The carbonates may be divided into two 

 families — the calcium-magnesium carbonate family and the iron-bearing 

 carbonate familv. The oxides are divided into two families — the iron-oxide 

 family and the silica family. 



