858 W. H. WEED — COAL FIELDS OF MONTANA. 



Although the Cinnabar field is quite small and the coal-measures are 

 scattered and broken, so that very extensive workings are not possible, yet 

 as the measures yield an excellent quality of coal [the portions now worked 

 will no doubt be thoroughly exhausted before the field is abandoned. 



The Bozeman Coal Field. 



Location, Extent, and General Geology. — Some 40 miles north of the 

 Cinnabar coal field is the so-called Bozeman field, the best-known and l6ug- 

 est-Avorked coal field of Montana. Although the coals of this field were 

 known in 1871 and were examined by the geologists of the Hayden survey,* 

 no exploitation was attempted until after the organization of the Northern 

 Transcontinental survey, for whom Mr. George H. Eldridgef made a careful 

 examination of the field, resulting in the purchase and working of the most 

 promising portions at Timberliue and Cokedale. 



In its entire extent, the Bozeman coal field embraces the foot-hills lying 

 at the northern base of the Boulder mountains and their continuation in the 

 Snowy range westward to Livingston and over the Missouri- Yellowstone 

 divide to Rocky canon, where the outcrops swing around and occur at the 

 eastern base of the Bridger range, having been traced northward some 25 

 miles. The field thus occupies the angle between the northerly trending 

 Bridger mountains and the east-and-west range of the Snowy and Boulder 

 mountains. 



The general geological structure is that of a large synclinal basin whose 

 southern and western borders are the mountains just mentioned. These 

 ranges are formed of steeply upturned Mesozoic rocks, resting conformably 

 upon Paleozoic limestones, w'hich lie at a steep angle upon metamorphic 

 gneisses. The chief feature of the region is a sharp folding of the sedimen- 

 tary rocks, in general parallel to the line of contact with the Archean. 



There is generally a strike fault, or its equivalent fold, running along the 

 range before the beds flatten out toward the lower country on the north and 

 east. 



In the angle formed by the meeting of the Bridger mountains and the 

 eastern ranges, this structure is disturbed by three sharp anticlinal folds, 

 whose axes pitch steeply toward the north ; so that while erosion has ex- 

 posed the Carboniferous rocks on the ridges the productive (Mesozoic) coal- 

 measures curve uninterruptedly around their ends. The western anticlinal 

 is cut at right angles by the picturesque gorge of Rocky caiion, through 

 which the Northern Pacific railway passes to the Gallatin valley. The 

 larger streams issuing from the mountains, particularly the Yellowstone and 



*lst Ann. Rep. U. S. Geol. and Geog. Surv. Terr., 1871, p. 46; 2nd Ann. Rep., 1872, p. 113. 

 t Report 10th Census, vol. XV: Mining Industries, 1884, p. 739. 



