10 The American Geologist. Juiy, i89i 
skirts only liave been touched, and there is a vast area for future 
enterprise, whilst north and beyond it is 5-et another large and un- 
touclied field known as the Yampa. 
The coal of this (J rand River field has a further importance from 
its close proximity to the greatest iron deposits so far discovered 
in Colorado, such as those of the Iron King, at White Pine, about 
thirty miles from Gunnison City, and the Cumberland iron mine, 
in Pitkin county, also within fair distance of this great coal field. 
SOUTHWESTERN OR LA PLATA FIELD. 
Meanwhile in southwestern Colorado the Rio Grande railway 
had entered and discovered another large field, covering thousands 
of square miles, and extending from Colorado into New Mexico 
and Arizona, known as the La Plata field. Here is an immense 
amount of coal in bodies from seventy feet down to five feet in 
thickness ; yet over this gi'eat area the development is restricted 
to four or five small mines, one at Monero, another at Flowda, 
and three or four about Durango, doing a very local trade. This 
small development is due to the possession of much of the field 
by the Ute Indians, and to the fact that but one railroad pene- 
trates the region. The coal is bituminous, and over certain areas 
makes fine coke. 
Besides these large fields, there are several smaller isolated ones, 
such as one in North Park, with thick seams of lignitic coal, an- 
other in South Park, for some years developed by the Union Pacific, 
and another undeveloped field on "Tongue Mesa," near Monti'ose. 
NORTHERN COLORADO COAL FIELD. 
This field extends north from Boulder into the Laramie plains 
of Wyoming, and south to Colorado Springs. Its approximate 
eastern boundary is a line drawn north and south some fort}' miles 
east of the mountains. Thi.s field has long been developed in 
Boulder county by the Marshall, Erie and Louisville mines, also 
by mines at Golden and Ralston. Marshall is one of the oldest 
mines in the state, having l^een worked for at least twenty years. 
By a series of faults, accompanied by erosion, the coal has locally 
been lirought near to the surface at a gentle dip of 5 degrees. The 
principal seam is six to eight feet thick. The coal is a good, 
pure, high-class lignitic coal, not coking, but well adapted to all 
purposes where extraordinary heat is not required. A large area 
has l)een worked out, a good deal burnt out b}- spontaneous com- 
