750 GEOLOGY 



Tertiary in the same region. This change probably did not take 

 place everywhere at the same time, so that beds correlated with 

 the Montana series at one point, are perhaps the time equivalents 

 of beds classed as Laramie at other points. The series consists 

 primarily of sandstone, and shale, with some conglomerate; but 

 with these clastic formations there is much coal. Both shale and 

 coal are more abundant below than above, while in the upper part 

 of the series conglomerate is not rare. The materials of the Lara- 

 mie formation seem to have been derived principally from the 

 pre-Paleozoic rocks of the mountain region. The thickness of 

 the Laramie is estimated at 1,000 to 5,000 feet, exclusive of the 

 transition (Mesozoic-Cenozoic) beds to be mentioned below. In not 

 a few places there is an unconformity in the great group of strata 

 heretofore classed as Laramie, and there is difference of opinion 

 as to whether the part above this unconformity should be called 

 Laramie. The present tendency is to regard it as Eocene. 1 It 

 may be added that there is difference of opinion as to whether there 

 is one widespread, major unconformity at the horizon indicated 

 above, or whether there are diverse local unconformities. If the 

 former, it should have great weight in determining the classification. 

 The latter would be less significant. 



In a considerable area of northeastern Wyoming, and in a largo 

 area farther north, 2 some of the Laramie lignite has been burned in 

 the ground. The burning was relatively recent, and locally is 

 still is progress. The firing appears to have taken place at the 

 outcrops on hill and valley slopes. The burning was accompanied 

 by fusion, semi-fusion, and baking, resulting in lava-like slag and 

 brick-red banks of indurated clay. The slag, etc., has had a notal >lo 

 effect on the details of the topography (Fig. 507) developed by wind 

 and water, and the color effects of the burning are striking. 



Coal. The Cretaceous is pre-eminently the coal period of the 

 west. Coal-beds occur in every one of its principal divisions in 



1 The Laramie question is well reviewed by Cross, Washington Acad. of 

 Sci., Vol. XI, pp. 27-45, 1909. Other recent discussions by Veatch an- Ion ml 

 in Am. Jour. Sci., Vol. XXIV, 1907, and Jour. Geol., Vol. XV, 1907. 



2 Allen, Proc. Boston Soc. Nat. Hist., Vol. XVI, p. 210. L874; also 

 Bastin, Jour. Geol., Vol. XIII, p. 408. These phenomena were notr.l :m<i 

 correctly interpreted by Lewis and Clark. See report of their expedition. 



