May, 1S45.] 237 



they appear in many places more like the work of art than nature. They are 

 usually called Clay hills by travellers, but repeated inspection has satisfied me 

 that clay occcupies but a small number of the strata, at least in the vicinity of 

 Fort Union. There are strata of sand, clay, shale, sandstone and coal alternat- 

 ing, but without regularity. I have frequently seen a stratum of sand com- 

 pletely covered by the washing from a superincumbent stratum of clay, like a 

 coat of paint, so that its nature could only be ascertained by scraping off the 

 thin covering of clay. I am inclined to think, however, that the proportion of 

 clay is greater in the lower strata of this series, immediately overlying the Nicol- 

 let clay. The cause of the singular castellated appearance of these hills is the 

 great variety of materials of which the strata are composed being acted upon in 

 various degrees by atmospherical influences, according to their capability of re- 

 sisting them ; hence the spires, pyramids, cones and other forms cut out by the 

 torrents, are frequently capped by hard sandstone, while the stratum, immediately 

 underlying it, may be sand or clay, which being more easily affected by the 

 atmosphere, crumbles away and leaves the sandstone projecting, and so on with 

 thirty or forty distinct strata of hard and soft sandstone, sand, clay, shale and 

 coal, of very marked shades of colour, and in many places reddened by the 

 action of fire. I should have remarked that the visible coal seams which com- 

 mence in the upper part of No. 4, are most abundant soon after entering into 

 No. 5, and become small and few in number about the Yellowstone. I counted 

 in one place eight seams of coal between the river bank and the top of the Bluff, 

 varying from six inches to four feet in thickness. This coal is very light, and 

 ignites with difficulty, emitting a very unpleasant odour while burning. Fossil- 

 ized wood is very abundant in No. 5 ; I saw one specimen very much flattened 

 by the pressure of overlying strata. Small beds of limestone are found at Fort 

 Union. 



In regard to the evidences of volcanic matter overlving the stratified rocks on 

 the borders of the Missouri, and of the existence of red pumice in such situa- 

 tions, I have no where been able to discover them, although I sought them dili- 

 gently in my frequent excursions into the Mauvaises Terrcs. The red appear- 

 ance of the shale and clay, and in many instances of the sandstone, is, I believe, 

 to be attributed to the action of fire, but may be more readily accounted for 

 from the effects of the spontaneous combustion of the coal going on at the pre- 

 sent day, than from volcanic agency. These evidences of fire occur in so many 

 of the strata at such different levels, that to give them the latter origin we are 

 compelled to suppose a succession of eruptions, but in this case what has become 

 of the tuffa and lava! It is impossible that I could have failed in discovering 

 some evidence of their existence. Still I do not pretend to deny that the pumice 

 which is so frequently found floated down the Missouri, may have had its origin 

 in these hills and may have been found there by Mr. Catlin ; (I do deny that it 

 is the cause of the red appearance of the hills) but may it not be accounted for 

 by the action of these spontaneous fires 1 May nut pumice be found without 

 volcanic action ? Some of the specimens of sandstone with impressions of 

 leaves, in the box sent to the Academy, are evidently changed in their specific 

 gravity by the action of fire, and your Committee will be better able than my- 



