538 THE CRETACEOUS PERIOD 



west of them through the last epoch of the Cretaceous period, but 

 most of the sedimentation was non-marine. Fresh-and-brackish- 

 water beds are widely distributed. 



The Laramie series records the transition from the marine con- 

 ditions of the Montana epoch to the fresh-water and land condi- 

 tions of the Tertiary in the same region. This change did not take 

 place everywhere at the same time. 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 thickness of the Laramie 

 series is estimated at 1,000 to 5,000 feet, exclusive of the transi- 

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

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

 formerly classed as Laramie, and there is difference of opinion as to 

 whether the part above this unconformity should be called Lara- 

 mie. The present tendency is to regard it as Eocene. 1 



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

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

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

 in progress. The firing appears to have taken place at the outcrops 

 on hill and valley slopes. The burning was accompanied by fusio'n, 

 semi-fusion, and baking, resulting in lava-like slag and brick-red 

 banks of indurated clay. 



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

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

 this part of the continent. The total amount of coal, chiefly in the 

 Laramie, is perhaps comparable to that in the Pennsylvanian sys- 

 tem, though the coal is not now so accessible, and its quality not 

 so good. It is estimated that along the east and west bases of the 

 Rocky Mountains there are more than 100,000 square miles of coal- 

 bearing lands, and Colorado alone is estimated to have 34,000,000- 

 ooo tons of available coal, 2 most of which is Cretaceous. The coal 

 is largely lignite, though in Colorado not a little of it has been 

 advanced to coking bituminous coal, and even to anthracite, where 



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

 Vol. XI, pp. 27-45, 1909. Other recent discussions by Veatch are found in Am. 

 Jour. Sci., Vol. XXIV, (1907), p. 18, and Jour. Geol., Vol. XV, 1907. See footnote 



P- 539- 



2 Storrs, 22d Ann. Kept., U. S. Geol. Surv., Pt. III. 



