266 
CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
1.14 in northern Texas, to a semianthracite in Arkansas, with a fuel 
ratio of nearly 9. The range in character of the coals in this field is 
shown in the accompanying table : 
Table showing fuel ratio* of coals in flic Southwestern field. 
Arkansas: 
Semianthracite 
Seinibituminous 
Bituminous 
Indian Territory : 
Bituminous . 
North Texas: 
Bituminous 
Number of 
analyses. 
27 
24 
7 
Minimum 
fuel ratio. 
5. 04 
3. 51 
1 . 26 
1.11 
Maximum ' Mean fuel 
fuel ratio. ratio. 
8.96 
7.62 
4.99 
5. 22 
5. 79 
2. 68 
1.45 
The Atlantic coasl Triassic <•< >al closely resembles the Carboniferous 
coals of the Appalachian field, but is generally higher in ash and 
sulphur. In the Richmond area the fuel ratios range from 1.8 to 3.4, 
the average of "analyses given by Mr. Woodworth being 2.4.° In the 
Deej) River area they range from 1.6 to 3, the average of 17 analyses 
being 2.11. 
In the Rocky Mountain and Pacific fields the coal presents very 
great diversity in character, the same basin sometimes containing all 
the intermediate varieties between Lignite, with a fuel ratio less than 
1, and anthracite with a ratio of 20 or more. These abrupt changes 
in chemical composition and physical properties are due to the vary- 
ing degrees of alteration which the coal lias undergone. The altera- 
tion is produced by the pressure due to the weight of overlying strata 
or to the folding of the strata by mountain-building forces and by the 
metamorphism of intrusive igneous rocks. The first of these agen- 
cies, vertical pressure, is least effective, but most widespread in its 
effects; the second, lateral pressure, is more effective and relatively 
local, while the third, intrusion, produces effects which are extremely 
localized and correspondingly intense. As a result of these condi- 
tions the coal of the plains region is largely lignite, although the 
lowest beds, those which have been most deeply buried, approach 
most nearly to true coal. Along the flanks of the mountains and in 
the interior basins, where the inclosing strata have been moderately 
folded, the coal is chiefly bituminous. In the same regions more 
intense folding and the intrusion of igneous rocks have converted the 
bituminous coal into seinibituminous or anthracite. 
"Twenty-second Ann. Rept. U. S. Geol. Survey, Pt. Ill, p. 37. 
