FOLIATED ROCKS 295 



Marble is an exceptional case of a completely crystalline rock 

 derived from sediments by dynamic metamorphism, which is not 

 foliated or schistose. This is believed to be due to the capacity 

 of calcite to recrystallize freely after it has been subjected to com- 

 pression and mashing. 



The economic value of the marbles makes them largely sought 

 after ; in this country they are extensively developed along the 

 Appalachian region, from Vermont to Georgia, in the Rocky Moun- 

 tains, and the Sierra Nevada. 



The Ophicalcites are crystalline magnesian limestones and dolo- 

 mites, with varying amounts of included serpentine, which gives 

 them a mottled appearance. They are not thoroughly understood, 

 and it appears that they may be formed in various ways. Some 

 ophicalcites are almost certainly marbles, in w T hich inclusions of 

 olivine, pyroxene, or hornblende have been formed and afterwards 

 altered into serpentine (see p. 22). Others would appear to be 

 broken and fissured serpentines, having the crevices filled up with 

 calcite deposited from solution. 



Anthracite is usually regarded as a metamorphic form of coal, 

 and, as we have seen in a preceding paragraph of this chapter, it 

 is formed from bituminous coal by contact metamorphism. On a 

 large scale it occurs chiefly in areas of folded and disturbed rocks, 

 though not invariably so. A more intense metamorphism of car- 

 bonaceous material gives rise to graphite (or black lead), a semi- 

 crystalline form of carbon, which, however, is a mineral rather 

 than a rock. 



B. FOLIATED ROCKS 



The foliated or schistose rocks are those which are divided into 

 rudely parallel planes, with rough or undulating surfaces, due to 

 the flakes and spangles of some mineral. The planes of foliation 

 may coincide with the original bedding planes or they may inter- 

 sect the latter at any angle, just as do the planes of cleavage and 

 fissility. The foliated rocks represent the most advanced stage of 

 what we can confidently call metamorphism, and may be derived 

 from either sedimentary or igneous originals ; it is not always pos- 

 sible to say which. 



