KINDS OF ROCKS. 71 



a. First. As to structure : whether — 



Massive, like sandstone, or granite, breaking one way about as easily as 

 another. 



Schistose or laminated, breaking into slabs, like flagging-stone; schistose is 

 usually restricted to the crystalline rocks, like gneiss and mica schist. 



Slaty, breaking into thin and even plates, like roofing-slate. 



Shaly, breaking unevenly into plates, and fragile, like the slate or shale of the 

 eoal formation, the Utica slate, etc. 



Concretionary, having the form of, or containing, spheroidal concretions; 

 some varieties are also called globidxferous, when the concretions are isolated 

 globules and evenly distributed through the texture of a rock. 



b. Second. As to hardness and firmness : 



Compact, or well consolidated. 



Friable, or crumbling in the fingers. 



Porous, so loose or open in texture as to absorb moisture readily. 



Uncompacted, or like loose earth. 



Flinty, very hard, and breaking with a smooth surface like flint. 



c. Third. As to the rock or mineral nature of the constituents : 



Granitic, like granite, or made of granite materials. 



Siliceous, consisting mainly of quartz. 



Quartzose, same as siliceous ; but also consisting largely of quartz in grains, 

 — a quality expressed by arenaceous, when the rock is mainly made up of quartz 

 grains. 



Micaceous, characterized eminently by the presence of mica. 



Calcareous, of the nature of limestone, or containing considerable carbonate 

 of lime, as a calcareous rock, a calcareous mica schist. 



Argillaceous, having a clayey nature or constitution, or containing much clay, 

 as shale is argillaceous, a sandstone may be argillaceous. 



Ferruginous, containing oxyd of iron ; sometimes having a red, brownish-red, 

 or brownish-yellow color in consequence of the disseminated oxyd of iron ; 

 sometimes containing the ore in plates or masses of a metallic lustre. 



Pyritiferous, containing pyrites (p. 64) disseminated through the mass, either 

 in cubic crystals, or in grains or masses. 



Basaltic, made of material derived from basalt; also like basalt. 



Pumiceous, made of pumice. 



Garnetiferous, containing garnets. 



So, also, staurotidiferous, containing staurotide; anthophyllitic, containing 

 acicular hornblende of the variety anthophyllite. 



Sedimentary rocks differ, further (d), as to the mechanical condition 

 of the constituents : whether — 



(a) Pounded stones or pebbles; or (b) angular stones; or (c) sand; or 

 (d) clay. 



Crystalline rocks differ, further, — 



