METAMORPHISM 297 



VARIOUS CLASSIFICATIONS AND NOMENCLATURES 



From the foregoing sketch of the processes of rock-making it 

 will be understood that the varieties of rocks are many, and that 

 they may be defined, named, and classified on many different bases; 

 for example: 



(1) If the mode of origin is chiefly in mind, rocks may be classed 

 as igneous (lavas, tuffs, etc.); metamorphic (schists, gneisses, an- 

 thracite, etc.); and sedimentary. The last includes (a) aqueous 

 (water-laid sediments, travertine, etc.); (b) eolian (wind-blown 

 sand and dust); (c) glacial (deposits by glaciers); (d) organic (peat, 

 coal, etc., and indirectly, limestone, infusorial earth, etc.). 



(2) On the basis of textural character, rocks are designated ve- 

 sicular (pumice, scoria, etc.); glassy (obsidian); porphyritic (p. 248); 

 granitic or phaneritic (pp. 248 and 262); compact, porous, earthy, 

 arenaceous (sandy), schistose, etc. 



(3) If chemical composition is to be emphasized, they may be 

 classed as siliceous, calcareous, carbonaceous, ferruginous, etc.; or, 

 in case of igneous rocks, as acidic, basic, or neutral. 



(4) If crystallinity is made the basis, igneous rocks are desig- 

 nated phanerites (crystals distinct), aphanites (crystals very small), 

 porphyries, glasses, etc. 



(5) On the basis of mineral composition, rocks are quartzose, 

 micaceous, chloride, pyritiferous, garnetiferous, etc. 



(6) Regarded as mineral aggregates, some rocks are simple and 

 some complex. If simple, they are named from the dominant 

 minerals, as dolomite, hornblendite, etc.; if complex, they take spe- 

 cial names, as syenite, gabbro (pp. 260, 262), etc. 



(7) On the basis of structure of the mass, rocks are classed as 

 massive, stratified, shaly, laminated, slaty, foliated, etc. 



(8) When physical slate and genesis are considered, they are 

 clastic, fragmental, or detrital, (conglomeratic, brecciated p. 263, 

 arenaceous, argillaceous (clayey), etc.); or pyroclastic (tufaceous, 

 agglomeratic, p. 263), etc. 



As one of these characteristics is most important in a given rock 

 or in a given study, and another in another, no one classification is 

 satisfactory in all cases. 



