STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 159 



aerial erosion are regarded as marking the presence of a great river, 

 meandering over broad flats. 



The coal is dull black with vitreous streaks and is brittle; but 

 the woody origin is still distinct and fragments of Sequoia are 

 abundant, associated with leaves of dicotyledonous plants. Trunks 

 and stumps, erect or prostrate and partially silicified, embedded in 

 the coal or projecting from the sandstone, are by no means rare. 

 Coal beds are usually less variable than the other members of the 

 section. The Healy coal of the Ulm group has been traced in an 

 area of about 600 square miles, but the name designates a horizon 

 rather than a coal bed. Where it is a single bed, it varies within 

 short distances from a few inches to 18 feet, but often it is repre- 

 sented by a series of beds in a vertical section of 50 feet. This 

 horizon is exceptional in extent, other beds, as a rule, having very 

 limited area. One, 15 feet thick, quickly thins to a few inches and 

 disappears ; often a bed thins away and another is seen in the section 

 at a little above or below its place. These are merely overlapping 

 lenticular deposits. Contemporaneous deposits of coal are frequently 

 separated by barren spaces. That these conditions, described in de- 

 tail by Wegemann, are characteristic throughout Wyoming is evi- 

 dent from the incidental references by other observers. 



Eocene deposits cover much of eastern Montana, extending 

 northward across the state from Wyoming into Canada and east- 

 ward into North Dakota. The isolated basins of eastern Montana 

 have been studied by several geologists. W^oodruff and Woolsey--* 

 examined fields on the western side of the area, where they observed 

 conditions hardly differing from those seen in Wyoming. Wood- 

 ruff states that the coal beds with maximum thickness of 5 to 10 

 feet were evidently formed in basins. Many of them have carbona- 

 ceous shale, at times containing streaks of lignite, as floor and roof; 

 at one mine he obtained Unio in the roof. Woolsey remarks that the 

 coal beds in his area are very irregular and are lenticular. Resin is 

 especially abundant in the Bull Alountain field, where the beds are 



224 E. G. Woodruff, "The Red Lodge Coal Field. Montana"; L. H. 

 Woolsey, " The Bull Mountain Coal Field," U. S. Geol. Survey, Bull. 341, 

 1909, pp. 94-97, 103, 104: 62-77. 



