MESOZOIC TIME — CKETACEOUS. 875 



latitude, or a third of the circumference of the globe, were undergoing 

 simultaneous orogenic movements, with like grand results. 



The deposit-making, preparatory to the Laramide system of ranges, 

 began, as has been stated, in the Cambrian, and went forward, with some 

 large interruptions, until the subsidence in the geosynclines of deposition 

 amounted to 25,000 feet. While the Laramie epoch was passing, there was 

 a deepening of 10,000 feet in some places during the Cretaceous period alone, 

 and in Montana over 7000 feet if the estimates of thickness are right. 



As once before stated, it is not supposable that the Archaean ridges 

 bounding such troughs participated in the great subsidence. Assuming the 

 load of sediments to have caused the sinking, in accordance with the isostatic 

 theory, the trough would have been made in the waters oft" the shores, and 

 would have been greatest a little distance out from the shores ; and the same 

 might be a consequence if lateral pressure were the cause of the subsidence. 

 The denudation of the ridges would have caused them to rise rather than 

 sink. 



The earlier movements connected with the upturning appear to have 

 begun before the Laramie depositions were completed, producing, according 

 to Cross, a small unconformity in bedding between the Lower Laramie and 

 the Denver beds, besides unconformity by erosion. The latter is described by 

 Weed as marking the junction of the Lower Laramie and the Livingston 

 beds. But the erosion-plane occurs at a level 200 feet below that of a 

 brackish-water bed, abounding in Oyster shells, like those of the Lower 

 Laramie, showing that true Laramie conditions still prevailed, and that the 

 erosion was an event of minor importance. If the orogenic work had actually 

 begun, violent currents in the water may have been produced where quiet 

 deposition had before been in progress ; and then great excavations of the 

 earlier-made beds may have been occasioned, followed by depositions of con- 

 glomerates and other coarse beds. Moreover, earthquakes and earthquake 

 waves from the adjoining sea may have been an agent in producing erosions 

 of the unconsolidated strata. 



The erosion at the base of the Upper Laramie has been supposed to amount 

 to several thousands of feet and to have taken place as a result of an eleva- 

 tion of the region to this height ; and this elevation has been thought 

 necessary for the supply of the Paleozoic material of the conglomerates. 

 But such a lift of the region would have changed the climate, and through 

 consequent river-erosion would have cut down the Laramie formation into 

 mountain valleys and ridges ; and it would also have exterminated the fauna 

 and flora; when, in fact, horned Dinosaurs existed after it, while the Denver 

 beds were in course of deposition, and their bones are associated with those 

 of various other Dinosaurs in regions not far distant. 



Igneous eruptions were also a feature of the early stages of the orogenic 

 movements, and also of its latest. The Wasatch, as described by King 

 (see map, page 360), had its outflows of trachyte chiefly from the region 

 of greatest wrenching between the main range and the Uinta plateau. 



