If this formation of sandstone, consisting of the two varieties just mentioned, ever extended across the valley of the Mississippi to the Alleghany mountains, as some might be disposed to believe, we cannot pretend to determine what was its position relative to the immense masses of fletz, limestone, and other rocks now found in that valley. But as the red variety is still extensively disseminated, and usually accompanied by those valuable substances, salt and plaister, it may not be amiss to trace, as far as our examinations have enabled us to do it, the outline of the region which it occupies. As we have before mentioned, it is found in the vicinity of the river Platte, in a highly inclined position, covering a narrow margin immediately at the foot of the Rocky Mountains. {288} From the accounts of Lewis and Clarke, we are induced to believe that it exists under similar circumstances, near the falls of the Missouri. On the Canadian it is constantly met with, from the sources of that river on the borders of New Mexico, near Santa Fé, 106° west, until you arrive within a short distance of its confluence with the Arkansa, in long. 97° west. The waters of the Canadian, from flowing over the sandstone in question, acquire an intense red colour, and are so impregnated with muriate of soda and other soluble salts as to be unfit for use. This, we are credibly informed, is also the case with the waters of three small rivers tributary to the Arkansa, above the Canadian, on the same side; also with the waters of Red river. Hence the conclusion appears to be justified, that this rock extends from near the Arkansa on the north, to a point beyond Red river on the south; and from near the mouth of the Canadian, an unknown distance to the west. It is not unlikely it may exist about the sources and upper branches of the Rio Colorado of California, the Red river of Santa Fé, and the other Red rivers of New Mexico. Near the mountains, and for a great distance to the south and east of the High Peak, it is covered by the gray sandstone already mentioned. This gray sandstone is the uppermost of those horizontally stratified rocks which are seen in this region, possessing convincing evidence of their being the deposition of an ocean or lake of salt water.