580 



GEOLOGY. 



has been suggested that the anthracitic character of coal depends 

 on the length of time the vegetation was exposed before being buried 



Fig. 269. — Section showing positions and relations of the coal beds at a point in Indian 

 Territory. The black lines indicate coal beds. Length of section about 11 miles. 

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



by other sedimentary rocks. 1 This seems to demand that thick beds 

 of coal be more anthracitic at their bases than at their tops, or else 

 that the thin, slowly accumulated parts be more anthracitic than the 

 thick, faster-formed parts. Neither of these things seems to be gen- 



Fig. 270. Fig. 271. 



Fig. 270. — Diagram showing slight faulting in the coal beds of Iowa. Bloomfield 



shaft, Des Moines, la. (la. Geol. Surv.) 

 Fig. 271. — Step fault in Davidson mine, Jasper Co., la. (la. Geol. Surv.) 



erally true, though the lower part of a coal-bed is sometimes " harder " 

 than the upper. 



Other varieties of coal appear to depend on the conditions to which 

 the vegetable matter was subject, either before or after burial, and 

 on the degree to which the bituminizing process has been carried. 



Iron Ore. 



The iron ore of the Coal Measures occurs in layers like the coal, 

 or in the form of nodules which are often concentrated at a given hori- 

 zon, forming a nearly continuous layer. The ore is sometimes in 

 the form of the carbonate of iron (siderite) and sometimes in the form 

 of oxide. The iron is often associated with clay, making clay iron- 

 stone. The iron ore of the Coal Measures seems to have been largely 

 deposited as a precipitate from the waters of inland and local basins, 



Stevenson, Jour, of Geol., Vol. I, p. 677. 



