250 Annals of the Carnegie Museum. 



seek ' ' between the mountains and the valleys ; then flows for some 

 distance through the mountains ; and finally, winding its way through 

 a narrow canon in the basalt twenty-five or thirty miles in length, 

 enters the great plain. It then turns more to the eastward and takes 

 a more nearly direct course to its destination in the^Gulf of Mexico. 



In the Vicinity of Logan. 



x'\t Logan I rejoined Mr. Raymond, who, after spending a few days 

 near Glendive, where he obtained reptilian and invertebrate fossils, 

 had gone to Logan to make collections from the Palaeozoic rocks, 

 which are so excellently exposed in this region. Here in an hour's 

 walk, one may travel over various horizons of Algonkian, Cambrian, 

 Devonian, Carboniferous, Jurassic, Cretaceous, and Tertiary rocks. 

 This was one of Mr. Raymond's principal objective points. He had 

 been successful beyond our best expectations, and had collected large 

 numbers of fossils from many different horizons. We continued the 

 work of collecting with undiminished success, Mr. Raymond making 

 some very important discoveries. 



Northwest of Logan there occur Tertiary deposits, which, as in 

 other places in the state, occupy not only the larger valleys, but the 

 smaller depressions between the hills. A few fragmentary vertebrate 

 fossils of Miocene age were found here. It is evident that at one 

 time the Oligocene and Miocene deposits filled not only the broader 

 valleys, but covered the foot-hills and bases of the mountains, and the 

 streams flowed at a much higher level than at the present time. This 

 accounts for the fact that the rivers and smaller streams ignore the old 

 valleys and cut canons in older rocks, which often rise several hundreds 

 of feet above the bottoms of the old valleys. The conditions were 

 perhaps similar to what they now are in Nevada, where the bases of 

 the mountains are buried in more recent deposits and the upper por- 

 tions project above the level plains. The rivers, when their beds 

 were lowered, maintained the general course of their channels through 

 hard and soft rocks. 



It has been supposed that uplifts across the paths of drainage pro- 

 duced separate lakes in the various valleys among the mountains, and 

 that these lakes were filled with sediments which were brought in by 

 streams. But on more extended examination these imaginary barriers 

 vanish. The Tertiary deposits, portions only of which appear to 

 have been formed in lakes, and which were deposited at several 



