450 GEOLOGY. 



igneous (lavas, tuffs, etc.); metamorphic (schists, gneisses, anthracite, 

 magnetite, etc.); aqueous (water-laid sediments, stalactites, 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 or structural characters are in mind, rocks are desig- 

 nated vesicular (pumice, scoria, etc.); rhyolitic (flow-structure rocks); 

 glassy (obsidian, tachylite); porphyritic (distinct crystals in obscure 

 matrix); granitic (well-grained); compact, porous, earthy, arenaceous 

 (sandy), schistose, etc. 



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

 classed as silicious, 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 designated 

 phanerocrystaUine (distinctly crystalHzed), microcrystalhne (minutely 

 crystaUized), cryptocrystalHne (hiddenly crystalHzed), and amorphous 

 (non-crystalline) . 



(5) If attention is fastened on certain ingredients, rocks are character- 

 ized as quartzose, micaceous, chloritic, talcose, pyritiferous, garnetifer- 

 ous, etc. 



(6) When rocks are regarded as mineral aggregates, if (a) the 

 aggregates are simple, they are named from the dominant minerals, as 

 dolomite, hornblendite, garnetite, anorthite, etc.; and if ih) the aggre- 

 gates are complex they take special names, as syenite (orthoclase and 

 hornblende), gabbro (plagioclase feldspar and pyroxene), etc. 



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

 as massive, stratified, shaly, laminated, slaty, foHated, schistose, etc. 



(8) When physical state or genesis is considered, they are grouped 

 as clastic, fragmental, or detrital (conglomeratic, brecciated, arenaceous, 

 argillaceous, etc.); or pyroclastic (tufaceous, scoriaceous, agglomeratic) ; 

 or massive, in a sense slightly different from that above (7) . 



As sometimes one of these characteristics and sometimes another is 

 most important in a given rock, or in a given study, no one classification 

 is satisfactory in all cases, yet each has its advantages in particular cases. 



