602 abstracts: geology 



don (Smilax), and the remaining 16 dicotyledons. They are entirely 

 unlike anything now living in the New World and appear to find their 

 closest relatives among species now living in Polynesia. 



From the facts now available it appears unquestionable that at 

 least the major portion of the Frontier formation was laid down in 

 fresh water. The climate during Frontier time appears to have been 

 tropical or subtropical. R. W. Stone. 



GEOLOGY. — The Bull Mountain coal field, Musselshell and Yellow- 

 stone counties, Montana. L. H. Woolsey, R. W. Richards, and 

 C. T. Lupton. U. S. Geological Survey Bulletin 647. Pp. 218, 

 with maps, sections, and illustrations. 1917. 



The Bull Mountain coal field came into prominence about 1906, 

 when the westward extension of the Chicago, Milwaukee & St. Paul 

 Railway was constructed through this part of Montana. This field 

 is the chief source of fuel for this new transcontinental road and in 

 consequence has been developed very rapidly, especially in the vicinity 

 of Roundup. 



The important coal-bearing formation is the Fort Union, which 

 belongs to the lower part (Eocene series) of the Tertiary system. The 

 rocks of this formation consist principally of massive, buff to yellow- 

 ish-gray sandstone and light-colored clay shale. The lower 200 to 

 300 feet of the formation contains little coal. 



The lowest formation outcropping in the Bull Mountain field is the 

 marine Bearpaw shale, which is recognized in this part of Montana as 

 the upper formation of the Montana group. The Lance formation 

 overlies the Bearpaw shale and appears to be in conformable sequence 

 with it. The Lance formation consists of alternating beds of yellow- 

 ish-gray sandstone, drab, yellow, and gray clays, clay shale, and grayish 

 sandy shale. Sandy material predominates. The lower part of the 

 formation is noticeably micaceous; the up" per part contains thin beds of 

 coal. 



The strata 200 to 300 feet thick immediately overlying the Lance 

 and apparently conformable both with the Lance below and the upper 

 part of the Fort Union above are known as the Lebo shale member. 

 The member is conspicuous on account of its dark or olive-green color 

 and the absence of the resistant sandstone which characterizes the over- 

 lying beds of the upper part of the Fort Union and the underlying Lance 

 formation. 



