262 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
the Richmond and Deep River areas. The strata of these basins rest 
directly upon the crystalline rocks of the Piedmont Plateau. They 
may originally have been continuous and nearly horizontal, but are 
now separated and considerably folded and faulted. They have also 
been invaded by dikes and sheets of igneous rocks, which have at some 
points converted the coal into natural coke or carbonite. While the 
coal is in some places of excellent quality, it shows great irregularity, 
as would be expected from the conditions under which it was depos- 
ited and the movements to which it has subsequently been subjected. 
These fields are chiefly of historic interest, since the first systematic 
coal mining in the United States was carried on within their borders. 
Cretaceous coalfields. — As conditions had been favorable for the 
accumulation of coal in the region east of the one hundreth meridian 
during Carboniferous time, so thej r were favorable for its accumula- 
tion during Cretaceous time in the region between the one hundredth 
and one hundred and fifteenth meridians. Since the deposition of the 
Cretaceoiis formations in this region it has been subjected to the 
action of mountain-building forces and to intense volcanic activity. 
Hence the coal-bearing formations, which may originally have been 
continuous over much of this region, occur in small, irregular basins 
separated by larger areas of elevation and erosion or by areas of igne- 
ous rocks. Although the folding of the strata and their invasion bj^ 
igneous rocks have greatly reduced the area of the coal-bearing forma- 
tions, the quality of the coal has been thereby greatly improved. In 
the extensive undisturbed Cretaceous areas which extend eastward 
from the Rocky Mountains beneath the plains region in Montana, 
Wyoming, and the Dakotas, there are numerous beds of lignite, while 
the same horizons on the flanks of the mountains yield high-grade 
bituminous coal. 
The Cretaceous coal fields are included within a belt that extends 
from the Canadian boundary southeastward for a distance of 1,200 
miles. Its axis coincides with the main range of the Rocky Moun- 
tains, but includes also numerous outlying ranges. Its greatest 
breadth is about 500 miles. It embraces portions of Montana, South 
Dakota, Wyoming, Colorado, Utah, and New Mexico. Mr. Storrs has 
described 45 separate areas within this belt, having an aggregate extent 
of 43,010 square miles/' All of these areas are known to contain work- 
able coal, but many of them are undeveloped and practically unex- 
plored, so that estimates of the productive area are not by any means 
exact. 
Two small areas of Cretaceous coal-bearing formations in western 
Texas properly belong with the Rocky Mountain fields. The west- 
ernmost of these is the San Carlos coal field, in El Paso County. 
Considerable outlay has been made here in development, but all the 
a Twenty-second Ann. Eept. U. S. Geol. Survey, Pt, III, p. 422. 
