202 



and Niobrara Rivers, are singularly alike, indicating their derivation from 

 the same source. The metamorphic rocks all over the West are com- 

 posed mostly of quartz and feldspar, the former predominating. It is 

 from the decomposition of these rocks that most of the Lacustrine sedi- 

 ments are derived. Judging from the character of the sediments, there 

 were no violent currents in the channels of the streams in Lacustrine 

 times, unless near their source. The sediments do not seem to differ 

 essentially in fineness where they lap over the flanks ot the granite 

 hills at the upper side of the basin. 



The fllling-up of the basin also caused the channel above to be full, so 

 that the fine matter would be distributed over the basin very equably, 

 and settle quietly at the bottom, as in any of our small fresh-water lakes 

 of the i)resent time. The thickness of the deposits in this basin maybe 

 estimated at from 1 200 to 1,500 feet. The height of the divide between 

 Madison and Gallatin Elvers ranges from 600 to 1,000 feet above the 

 beds of those streams, so that it is probable that the estimate of the 

 aggregate thickness as 1,500 feet is rather under than over the true 

 one. 



The writer was unable to find any of the vertebrate remains in this 

 basin that have usually been obtained from this formation in many 

 other places. Many persons living in the country informed me they 

 had seen remains of turtles, fragments of skulls, and bones of other 

 animals in various places, but the time at my disposal did not permit 

 me to make a careful search. In the side gorges, or. gullies, of the Madi- 

 son, we found most beautiful si)ecimens of silicified wood in great quan- 

 tities, some of which might be said to be opalized. 



In the valley of the Jeflerson, near its source, there is one of these 

 small expansions of the valley, in which there is a considerable thick- 

 ness of the Lacustrine sediments. In 1871, I found there a species of 

 Helix, and the jaws of a vertebrate animal of the genus Anchitheruwi. 



In the American Journal of Science for February, 1876, Messrs. Grin- 

 nell and Dana discovered a lake-basin near Camp Baker on Dry Creek, 

 which is evidently one of great interest. In this basin they seem to 

 have found a variety of vertebrate remains, representing two epochs, 

 Miocene and Pleiocene. I am not aware that the lower beds of the 

 White River group had been previously observed in Montana. It is 

 quite possible, as the gentlemen suggest, that the Pleiocene lake on 

 Deep River was connected with those near Fort Ellis and the Three 

 Forks. We may, with perfect confidence, connect them all, for they can 

 be traced with very short interruptions from the sources of the Madison 

 and Yellowstone Rivers and their branches to the points where the 

 rivers leave the mountain-districts for the plains. It may be re- 

 marked here that these peculiar Lacustrine deposits are found for the 

 most part only in the mountainous portions ; that in the plains, if they 

 ever occur at all, they are of older date. In the valley of the Sweetwater 

 River there are isolated patches of the Pleiocene marls distributed over 

 the Miocene deposits, very similar to those on Deep Creek ; but there are 

 here low granite ridges on either side, showing that tlie foundation-rock 

 on which the modern Tertiary deposits were laid down in that region 

 is granite or gneiss. These same modern Lacustrine deposits occur in 

 the North and Middle Parks, Colorado. 



The lower section of Plate VII, " The Second Valley of the Yellow" 

 stone,'' is an illustration of one of the oval Lacustrine basins so com" 

 mon along the rivers. It is about thirty miles long, and will average 

 about thee miles in width. The lake-sediments have been swept out of 

 the basin to a great extent, but there are quite large remnants remain- 



