. Annual report. 1st-12th, 1867-1878. Geology. GEOLOGICAL SURVEY OF THE TERRITORIES. 113 are now the valleys of the Jefferson, Madison, and Gallatin Rivers, reach- ing to the junction of the three streams. The hills between the Gal- latin and Madison, and between the Madison and Jefferson, are Plio- cene, remnants of the same beds, and when we ascend the mountains on the southern border of this old lake-basin the whole plan lies spread out before us. Each of the rivers has cut deeply into these Pliocene rocks, and their valleys are the results of the erosion that has taken place since the drai
. Annual report. 1st-12th, 1867-1878. Geology. GEOLOGICAL SURVEY OF THE TERRITORIES. 113 are now the valleys of the Jefferson, Madison, and Gallatin Rivers, reach- ing to the junction of the three streams. The hills between the Gal- latin and Madison, and between the Madison and Jefferson, are Plio- cene, remnants of the same beds, and when we ascend the mountains on the southern border of this old lake-basin the whole plan lies spread out before us. Each of the rivers has cut deeply into these Pliocene rocks, and their valleys are the results of the erosion that has taken place since the draining of the ancient lake. The question of priority of elevation of the Bridger liange, and that into which Spring Canon is cut, is one of some interest. That there has been more than one force at work to give the surface its present configuration is evident. The question is, whether or not they acted synchronously. The forces that elevated the layers of Spring Caiiou ard those of Bridger were, I take it, entirely distinct from each other and separated by long periods of time. The Bridger Kange was the first to be elevated, and its eleva- tion occurred probably about the end of the Cretaceous period, and be- fore the beginning of the Eocene, while the range running south from Spring Canon was elevated some time after the Eocene and prior to the deposition of the Pliocene strata. This is proved by the foot that the Cretaceous rocks on one side of the valley are conformable to the Bridger Ptange and on the other side to the Spring Canon layers, while the Eocene rocks are conformable only to the latter, and the Plio- cene rocks have been affected by neither range. • I believe also that the Fig. 27 SECTION FROM SPRING CANON TO DRIDGER PEAK. A, Bridgor Peak; B, Spruit; Ciiuon ; C, Tertiai-y ; D, Cretaceous; E, Juiassic ; r, Carboniferous; G, Drift; H,'Tertiary hills. elevation of the Bridger Eange was the more gradual of the two. The eleva- tion of the Spring Canon layers was &
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