GEOLOGICAL PAPERS. 14T 



gypsum beds are found upturned at an angle of 90 degrees. The 

 Trias and Jura beds do not fall short of 1000 feet, or one-half that of 

 the Carboniferous, in thickness. 



The system most frequently exposed is the Cretaceous. All the 

 formations from the Dakota to the Laramie are represented. There 

 is much of interest in connection with each, but space in this paper 

 does not allow details of those below the last named. It is the Laramie 

 that is the most interesting, and that has the greatest economic value. 

 Its thickness is one-third that of the entire Cretaceous. It is also 

 widely developed. It seems to be wholly a fresh-water deposit, lacus- 

 trine or estuarine, and paludine. Its beds are sandstone, shale or 

 clay, and coal, with later intervening plutonic intrusives. The Lara- 

 mie period was a long one, and its climate was temperate. The 

 scarcity of limestone precludes deep seas ; the absence of gypsum in- 

 dicates that no wide marine, littoral basins were overflowed by the 

 tides, and their waters left to be evaporated by the sun. But the 

 flora is suflBcient evidence of the character of the aqueous depository. 

 Little of sulfur or iron is found, which argues a period of plutonic 

 inactivity and a drainage area devoid of extensive volcanic products. 

 A series of great lakes, probably, extended from Mexico, or perhaps 

 the City of Mexico, to southern Alaska, perhaps the longest chain of 

 lakes in geological history. Their outflow was probably into the 

 Pacific, principally through the Columbia and Colorado rivers. The 

 area of erosion was largely one of granitic and metamorphic rocks, 

 with much quartzite. Under continued deposits, mostly silicious 

 sands and fine clays, the lakes became fens, or their marshy margins 

 were greatly broadened and became the habitat of dense coniferous 

 forests, the origin of the Laramie coal-beds ; a vegetation entirely differ- 

 ent from that which produced the coal-beds of Kansas. Through 

 progressive elevation, the outlets of the lakes were raised by degrees 

 exceeding the erosive effects of the clear lake water, and the flat forest- 

 covered lands about the lakes became overflowed, and the forests were 

 drowned out. The sand and silt which had heretofore been carried into 

 the lakes through drainage channels now began to be dropped in the 

 overflowed flats ; more of them than heretofore were carried through 

 the outlets, scouring these more rapidly. 



The sandy flats were once again but moist or swampy grounds. 

 Between raising the surface of the lowlands by sediment, and deepen- 

 ing the lake outlets by increased corrading material, the equilibrium 

 was restored and passed. Perhaps pauses in the elevating forces may 

 also have aided the effects. Resinous, rank conifers once more occu- 

 pied the drained surfaces and the material for coal-beds accumulated. 

 Alternately the conditions described succeeded each other, until scores 



