30th.--After traveling about twelve miles this morning, we reached a place where the Indian village had crossed the river. Here were the poles of discarded lodges and skeletons of horses lying about. Mr. Carson, who had never been higher up than this point on the river, which has the character of being exceedingly rugged, and walled in by precipices above, thought it advisable to encamp near this place, where we were certain of obtaining grass, and to-morrow make our crossing among the rugged hills to the Sweet Water river. Accordingly we turned back and descended the river to an island near by, which was about twenty acres in size, covered with a luxuriant growth of grass. The formation here I found highly interesting. Immediately at this island the river is again shut up in the rugged hills, which come down to it from the main ridge in a succession of spurs three or four hundred feet high, and alternated with green level _prairillons_ or meadows, bordered on the river banks with thickets of willow, and having many plants to interest the traveler. The island lies between two of these ridges, three or four hundred yards apart, of which that on the right bank is composed entirely of red argillaceous sandstone, with thin layers of fibrous gypsum. On the left bank, the ridge is composed entirely of silicious pudding-stone, the pebbles in the numerous strata increasing in size from the top to the bottom, where they are as large as a man's head. So far as I was able to determine, these strata incline to the northeast, with a dip of about 15°. This pudding-stone, or conglomerate formation, I was enabled to trace through an extended range of country, from a few miles east of the meridian of Fort Laramie to where I found it superposed on the granite of the Rocky mountains, in longitude 109° 00'. From its appearance, the main chain of the Laramie mountain is composed of this rock; and in a number of places I found isolated hills, which served to mark a former level which had been probably swept away.
These conglomerates are very friable, and easily decomposed; and I am inclined to think this formation is the source from which was derived the great deposite of sand and gravel which forms the surface rock of the prairie country west of the Mississippi.