58 GEOLOGY 



VARIOUS CLASSIFICATIONS AND NOMENCLATURES 



From the foregoing sketch of the processes of rock-making 

 it may be inferred that the varieties of rocks may be almost un- 

 limited, and that they may be denned, 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.); aqueous (water-laid sediments, travertine, etc.); 

 eolian (dunes, loess in part) ; glacial (till, moraines) ; clastic (mantle- 

 rock, sandstone, conglomerate, etc.); organic (peat, lignite, coal, 

 etc., and indirectly, limestone, chalk, infusorial earth, etc.); and 

 so on. 



(2) If the textural characters are in mind, rocks are designated 

 vesicular (pumice, scoria, etc.); glassy (obsidian); porphyritic 

 (distinct crystals in obscure matrix) ; granitic (distinctly grained) ; 

 compact, porous, earthy, arenaceous (sandy), schistose, etc. 



(3) If the chemical composition is chiefly regarded, they may 

 be classed as siliceous, calcareous, carbonaceous, ferruginous, 

 etc.; or, if the chemical nature is considered, they are grouped as 

 acidic, basic, or neutral. 



(4) If the crystalline character is made the basis, they are desig- 

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

 and amorphous (non-crystalline). 



(5) If attention is fastened on certain ingredients, rocks arc 

 characterized as quartzose, micaceous, chloritic, talcose, pyritif- 

 erous, garnetiferous, etc. 



(6) When rocks are regarded as mineral aggregates, the aggre- 

 gates may be simple or complex. If simple, they are named from 

 the dominant minerals, as dolomite, hornblendite, garnetite, 

 anorthite, etc.; if complex, they take special names, as syenite, 

 gabbro (p. 29), etc. 



(7) When the point of view is structure of the mass, they arc 

 classed as massive, stratified, shaly, laminated, slaty, foliated, 

 etc. 



(8) When physical state and genesis are considered, they arc 

 grouped as clastic, fragmental, or detrital (conglomeratic, brec- 



