212 A TREATISE ON METAMORPHISM. 



Following the ordinary classification, the abundant rock-making 

 constituents may be considered under the headings: Native Elements, 

 Sulphides, Fluorides, Oxides, Carbonates, Silicates, Titanates, Phosphates, 

 and Sulphates. 



NATIVE ELEMENTS. 



GRAPHITE. 



Graphite: 



Crystallized carbon (C). 



Rhombohedral. 



Sp. gr. 2.09-2.23; av. 2.16. 



occurrence — Graphite occurs as a very widely disseminated constituent 

 in the extremely metamorphosed sedimentary rocks, which in their original 

 condition contained carbonaceous material. It is especially prevalent in 

 scales in the marbles, schists, and gneisses. In some instances the original 

 beds were so heavily carbonaceous as to give considerable layers a large 

 percentage of which is graphite. Such layers are illustrated by the 

 graphitic shales of Worcester, Mass." Graphite occurs to some extent with 

 the very hard anthracite coals, a part of the carbon having- passed over to 

 the graphitic condition. Such graphitic coals occur in the Rhode Island 

 coal field. 6 The reaction producing graphite as a metamorphic mineral 

 recpiires great pressure and takes place with decrease in volume. This 

 mineral in the sedimentary rocks is therefore a product of the zone of 

 anamorphism. 



Graphite is said to occur as an original constituent in some basaltic 

 rocks. During the alterations of carbonaceous rocks the hydrocarbon com- 

 pounds, as gases, oils, and bitumen, wander widely in the solutions. In 

 some cases such compounds are deposited in the opening's of original rocks 

 Later these compounds may be altered to graphite, and yet the carbon not 

 be an original constituent of the magma from which the rocks crystallized. 



Alterations — Alterations of graphite are not recorded, but it is by no 

 means certain that this mineral is not very slowly oxidized tinder favorable 

 conditions in the belt of weathering. 



THE SULPHIDES. 



The sulphides which are important as rock-making minerals are 

 pyrrhotite, pyrite, and marcasite. Many other sulphides are important in 



a Perry, J. H., Note on a fossil coal plant found at the graphite deposit in mica-schists at Worcester, 

 Mass.: Am. Jour. Sci., 3d ser., vol. 29, 1885, pp. 157-158. 



&Shaler, N. S., Woodworth, J. B., and Foerste, A. F., Geology of the Narragansett Basin: Mon. 

 U. S. Geol. Survey, vol. 33, 1899, p. 82. 



