578 



GEOLOGY. 



situations where other sorts of rock are metamorphosed. These 

 phenomena long ago suggested that anthracite is to be looked upon 



\^r 



Fig. 265. — Section of the Western Middle anthracite coal basin of Pennsylvania. 

 Length of section, a little less than two miles. (Stoek, U. S. Geol. Surv.) 



as metamorphic coal produced from bituminous by processes similar 

 to some of those which induce metamorphism in other sorts of rock. 



Fig. 266. — Section south of Nanticoke, showing the position and relations of the 

 coal beds, in the Northern anthracite coal field. Length of section a little less 

 than two miles. (Stoek, U. S. Geol. Surv.) 



The fact that metamorphic coal is usually found in regions where 

 erosion has exposed its beds (Figs. 264 to 267) has led to the conjec- 



Fig. 267. — Section at Scranton. Length of section between 4 and 5 miles. 

 (Stoek, U. S. Geol. Surv.) 



ture that exposure may be a real factor in the problem, the exposure 

 favoring the escape of the volatile constituents, and so aiding in the 

 transformation of bituminous coal to anthracite. Beds of bituminous 

 coal are, however, often exposed in valley bluffs. 



Both dynamic action, involving pressure and heat, and exposure 

 would seem to be conditions favoring the development of anthracite, 

 but it does not follow that these are the only factors in the problem, 

 or that anthracite coal has never been produced in other ways. It 



