CHARACTERISTICS OF ROCKS. 63 



■crystalline grains, and not water-worn particles or fragments of other 

 rocks. Examples, granite, gneiss, mica schist, basalt. 

 The crystalline rocks may have been crystallized, — 



a. From fusion, like lava or basalt, when they are called igneous 

 rocks. Igneous rocks are often called intrusive rocks, a term signifying 

 that they have been ejected from below, through fissures intersecting 

 other rocks. 



b. From solution, as with some limestone. 



c. Through long-continued heat without complete fusion. By this 

 last method, sedimentary beds," that is, those made originally from 

 mud, clay, etc., have been altered into granite, gneiss, or mica schist, 

 and compact limestone into statuary-marble. 



Since, in such cases, a bed originally sedimentary has been meta- 

 morphosed into a crystalline one, rocks of this altered kind are called 

 metamorphic rocks. 



In the following descriptions, a separate subdivision is made of the 

 calcareous rocks or limestones, which are mostly sedimentary in original 

 accumulation, but generally lose that appearance as they solidify. 



Characteristics of Rocks. — Iudependently of the characters above 

 mentioned, rocks differ in kinds : — 



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 coal 

 formation, the Utica shale, etc. 



Concretionary, having the form of, or containing, spheroidal concretions; some va- 

 rieties are also called globuliferous, when the concretions are isolated globules and evenly 

 distributed through the texture of a rock; others are oolitic, when made of an aggrega- 

 tion of minute concretions, not larger than the roe of a fish, the word coming from the 

 Greek tide, egg. 



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. - 



Uncmnpacted, 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, containing much quartz. Quartzytic, consisting in part of quartzyte, as 

 quartzytic gneiss. Arenaceous, consisting of, or containing, quartz grains in a feebly 

 coherent condition. 



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. 



