482 THE CRETACEOUS PERIOD 



guished, although not everywhere separable ; the Fort Pierre, 

 which is composed of shales and sandstones with a maximum 

 thickness of 8000 feet, and the Fox Hills, sandstones and some 

 shales, which do not exceed 1000 feet. This movement of up- 

 heaval in the interior was accompanied or followed by an uplift 

 on the Atlantic and Gulf coasts, for along these borders the upper- 

 most Cretaceous beds are either wanting or represented by exceed- 

 ingly thin deposits. In the interior the continued upheaval caused 

 estuarine, fresh-water, and swampy conditions to prevail over very 

 wide areas, though not so widely extended as had been the Upper 

 Cretaceous sea. The older part of this great fresh- and brackish- 

 water formation is the Laramie. The northwestern part of the 

 continent had been converted into dry land, but a broad estuary 

 extended up the course of the present Mackenzie River to lati- 

 tude 6 2 N. Another and vastly larger body of water began 

 about latitude 57 N. and reached, though perhaps with interrup- 

 tions, to northeastern Mexico, surrounding the Colorado island. 

 This great inland sea was 2000 miles long and 500 miles wide, 

 though it is by no means certain that all of it was under water at 

 the same time. In the swamps and shallows were gathered great 

 quantities of vegetable matter, now converted into coal seams. 

 Workable coal is found in all the stages of the western Cretaceous, 

 but none of these stages is comparable to the Laramie for the 

 extent and thickness of its coal measures. In the Laramie sea 

 were alternating conditions of fresh and brackish water and, it is 

 said, occasional inroads from the ocean occurred. 



The Laramie was a time of tranquillity, with only slow and gentle 

 changes of level, but towards its close some important disturb- 

 ances took place, especially along the Rocky Mountains. The 

 first of these movements affected only the Colorado island, and 

 its effects are especially well shown in the Denver basin, where 

 some 800 feet of conglomerates (the Arapahoe) rest upon the 

 Laramie unconformably. The second series of movements was 

 much more extensively felt, producing marked unconformities both 

 in Colorado and Montana. In Colorado there was a great vol- 

 canic outburst, and the Denver stage, which overlies the Arapahoe 



