NUMBER OF COAL SEAMS. 325 



edges of the underlying coal measure sandstones and makes it extremely 

 difficult to outline the extent of the field. • 



A very brief but comprehensive account of the coal seams of this field 

 was published by J. E. Wolff* who visited the locality before the mines 

 were opened. 



Extent of the Field. 



Very little is thus far known of the extent of the Rocky Fork field. 

 Prospecting has been confined to the vicinity of the Red Lodge mines, 

 being chiefly done in the broken country to the eastward, where the deep 

 trenches of streams have exposed the coal seams. The coal has been 

 traced and found to be workable at least as far eastward as the Clarkes 

 fork bottom. Westward no prospecting whatever has been done, and 

 therefore the presence of the coal is not proved ; but the same geological 

 structure and the same rock series has been found by the writer to ex- 

 tend for many miles westward, and there is no reason to doubt the con- 

 tinuity of the coal measures in this direction, although prospecting with 

 drills will be necessary to prove the value of the land. 



The southern boundary must perforce be the fault line that runs along 

 the base of the mountains ; the northern boundary of the field is at a 

 variable distance of 3 to 4 miles from this fault line, according to the dip 

 of the beds. 



The Coal Measures. 



Structure. — The coal seams occur interbedded with coarse gray and buff 

 sandstones and thin clayey shales, such as characterize the coal rocks of 

 the Bozeman field. Fossil leaf remains are found in these sandstones 

 and rather well preserved shells of Undo in the slates over the coal. The 

 total number of coal seams is not known, but nineteen have been ex- 

 amined, of which six have been mined at Red Lodge. Of the nineteen 

 seams examined, eleven show over six feet of coal. 



About the town of Red Lodge the coal measures outcrop as heavy 

 sandstone ledges and shale belts on the eastern bluffs of the valley. The 

 prevailing dip is 15° southward or toward the mountains. The eroded 

 edges of the beds are covered by gravels known to be from 20 to 160 feet 

 thick in the vicinity of the town and forming the; surface of the level 

 benchlands in which the valley is cut. On the south the beds flatten out 

 toward the fault that brings the coal measures against the white lime- 

 stones of the Paleozoic. The coal rocks are cut by a dike of igneous 

 rock a mile above the town, a prominent ledge that was traced for sev- 

 eral miles to the eastward, trending southeasterly. The creek, issuing 



♦ Tenth Census, vol. w, Washington, 1886, p. 755, 



XLIV— Bl ii. < i i.i.i . Sor. Am., V.. i. :;. Iv.H. 



