434 FLORA OF THE LAKAMIE <;KOri'„ 



the Lower Mis.si.ssii)]n and Lower Kio (Iraiulc Valleys. That this {jreat 

 iiilaud sea spread over this entire territory is not at all disproved by 

 the absence of Laramie strata from large parts of it, since these parts 

 are situated, in most cases, in monntainons rejjions where the n])per 

 strata might be expected to have been generally eroded away. 



This Laramie sea existed during an immense period of time and was 

 finally but very gradually drained by the elevation of its bed, through 

 nearly the middle of which longitudinally the Rocky Mountains and 

 Black Dills now run. The exceeding slowness of this event is shown 

 by the fact, so clearly brought out by Dr. White, that the marine forms 

 of the Fox Hills strata, as they gradually found themselves surrounded 

 by a less and less saline medium on the rising of the intervening land 

 area, bad time to become transformed and adapted to brackish-water 

 existence, while these new-formed brackish-water species, ■when the sea 

 at length became a chain of fresh-water lakes, had time again to take 

 on the characters necessary to fresh water life. 



Dr. White recognizes the fact that the upheaval of the strata that 

 formed the bottom of this sea took place, not in one uniform jjrocess of ele- 

 vation, but in a prolonged series of rhythmic fluctuations of level, whose 

 algebraic sum constituted at length a mountain uplift. But the numer- 

 ous coal seams one above another that characterize the greater part of 

 these beds, and equally the successive dei)osits of vegetable remains at 

 different horizons, speak even more eloquently than any animal remains 

 can do of the oscillatory history of the bed of this sheet of water. 



There may have been, and doubtless were, as Major Powell believed, 

 many islands scattered over the surface of this sea in Laramie time, and 

 the evidence generally warrants us in assuming that a low, level country 

 surrounded the sea, with marshy and swampy tracts. The islands and 

 shores were heavily wooded with timber that can be as certainly known 

 in its general character as we can know the timber of our present for- 

 ests. But that for the greater part of the Laramie period there also 

 existed at no great distance a large amount of elevated land, there can 

 be no doubt. The deposits are chiefly siliceous in the southern districts 

 and argillaceous In the northern, but the nature of their deposition 

 poiTits unmistakably to the existence of large and turbulent rivers that 

 fell into the quiet sea and brought down from areas of rapid erosion 

 immense quantities of silt corresponding to the nature of the country 

 over which tliey flowed in their course. Where these elevated sources 

 of this abundant detritus were then located is one of the great problems 

 for the present and the fu' ure geologist to work out. 



The deposition of this material was almost always quiet, the particles 

 snsi)ended in the turbid waters of the streams silently settling from 

 the buoyant waters of the sea as fast as they became distributed about 

 the numths of the rivers, and thus embedding the leaves that periodically 

 fell in vast numbers into it. The marked absence of fiiiits, stems, and 

 other objects that possess considerable thickness shows that this was 



