PEALE.j RESUME PALAEOZOIC ROCKS — MESOZOIC ROCKS. 621 



trict. There was, therefore, no disturbance during the period within the 

 limits of the area studied by us during the season of 1877. The evidence 

 also points to a successive subsidence, which corresponds to what was 

 observed in Colorado. * 



Along the western edge of the Wind Eiver Mountains we find that 

 Tertiary strata rest on the Archaean rocks, and Mr. St. John informs me 

 that in the Teton Mountains we do not find the chloritic rocks of the 

 Cambrian interposed between the Potsdam sandstone and the Archaean 

 nucleus of the range. 



It appears, therefore, that in Cambrian time the shore line was a long 

 distance west of the Eocky Mountain chain. The upper part of the Car- 

 boniferous indicates a change from the deep seas to shallower seas, which 

 continue through the earlier portion of the Mesozoic time. The faulting 

 of the Palaeozoic rocks of the eastern ranges is Post-Cretaceous or Early 

 Tertiary in its occurrence, for we find the Jura-Trias and Cretaceous 

 conformable to the underlying Carboniferous. 



MESOZOIC EOCKS. 



TheMesozoic rocks of the district are particularly interesting, not only 

 in their lithological structure, but in their faunal characteristics, espe- 

 cially as regards the portion referred to the Triassic. They occur in long, 

 narrow zones, extending north and south along the edges of the Palasozoic 

 outcrops in the mountains; and east of the mountains, where the uplift has 

 not been so high nor the erosion so great, we find them folded and un- 

 derlying Tertiary strata, with which they are markedly uncomformable. 

 In the region of the Blackfoot the Mesozoic rocks have been subjected 

 to a complicated folding, and form important ranges, which extend to 

 the northward into Mr. St. John's district. The sequence of the strata 

 shows a progress from limestones to sandstones. The Triassic is an 

 alternation of limestones and arenaceous shales, with red sandstone at 

 the top. In the Jurassic we have limestones and shales, while in the 

 Cretaceous the limestones are few, the progress being from argillaceous 

 and calcareous shales to siliceous sandstones, which form heavy beds in 

 the Fox Hills Group. 



The Cretaceous formation is doubtless present above the Jurassic in 

 most parts of the district, but the data obtained are meager. Fox Hills 

 fossils were obtained at two localities, but the lower divisions were not 

 positively recognized. 



It has been frequently stated of late that the separation of the Trias 

 from the Jura is, at present, a matter of difficulty in the West, from the 

 fact that the forms supposed to be characteristic of the Jurassic have 

 been found in the lower rocks referable to the Trias. My collections 

 from the base of the Trias during the season of 1877 appear to point to 

 the same state of confusion in the distribution of the Triassic and Ju- 

 rassic fauna, although there are indications that data may yet be found 

 which will warrant the definite separation of the Trias from the Jura. 

 Either there is no line to be drawn between the two periods in this por- 

 tion of the country, or, as is more probable, we are not yet familiar 

 enough with the organic remains of the periods to say what are the 

 characteristic faunse. The period of sedimentation appears to be un- 

 interrupted from one period to the other, with the strata of both similar, 

 and it is scarcely to be wondered that some forms of the animal life of 

 the periods should also be continuous. I, therefore, still use the term 



* Report U. S. Geol. Survey for 1874, 1875, pp. 68, 69. 



