236 [May, 1845. 



lities named as the ending of one and beginning of another series, are not in 

 fact the beginning of the upper bed, but are the points where the superior stra- 

 tum of the last series is lost sight of in the River. Hence it is evident (from 

 the fact of the strata being perfectly horizontal) that while the lower series will 

 never be seen higher up the River than the points indicated, the superincumbent 

 series must have its beginning many miles lower down, according to the height 

 of the Bluffs and the angle of descent of the stream. 



Whenever we ascended the Bluffs to the High Prairies, we found them more 

 or less covered with a Boulder drift, with frequent denuded points, leaving the 

 larger Boulders lying upon the regular strata. As we ascended the River, the 

 Boulders, which are much worn and rounded, increased in size (with some vari- 

 ations) until we reached the high hills of the second Great Bend, between the 

 Mandans and the mouth of the Yellowstone, say, from the size of paving stones 

 at Fort Croghan to rocks of several tons at the second Great Bend, after which, 

 and at the Yellowstone, half a ton would probably be the size of the largest 

 Boulder. 



The exceptions to perfectly horizontal stratification are only found in series 

 No. 4, and appear to have arisen from two causes — from slides of the Bluffs, 

 and from the fact of their frequently taking fire and burning for several years, 

 causing the Bluffs to sink and crumble down. Mr. Bel) was the only one of 

 our party who had an opportunity of witnessing the burning of the cliffs, while 

 on a hunting excursion about 30 miles above the Yellowstone on the northern 

 bank of the Missouri. Smoke was issuing from the summit of the cliffs under 

 which he was riding — there was a thin seam of coal on a line with the smoking 

 part of the cliff, but where it was burning, and where the fire appeared (by the 

 crumbling down of the cliff) to have passed, there was no coal visible. I am 

 inclined, from this circumstance, and from having found remains of coal in a 

 stratum of sandstone containing impressions of leaves, and bearing evident 

 marks of fire, to think with Lewis and Clark, that the burning of the cliffs is to 

 be attributed to the spontaneous ignition of the coal. The soil in No. 4 and 5 is 

 strongly impregnated with salts, apparently sulphate of magnesia (7) so that 

 most of the brooks and springs are saliferous. In excursions on the Prairies it 

 is difficult to find good water. Soon after rains it may be found in small basins, 

 but these soon become brackish, unless they are in the detritus of the drift. 



Series Nos. 3 and 4 appear to contain the greatest quantity of crystallized salts, 

 such as alum, copperas and sulphate of magnesia, and saliferous springs and 

 streams in great numbers. Warm springs are said to exist, but we did not meet 

 with them. We found a substance like petrolium in colour and consistence, but 

 without odour. Crystallized gypsum (1 see specimens) is exceedingly abundant 

 in No. 4, lying in thin veins, cutting the regular stratification at right angles, 

 Fossil shells abounded most in No. 4. 



In No. 5. commence the remarkable strata which form the picturesque hills 

 noticed by travellers, and called the Mauvaises Terres by the trappers and voy- 

 ageurs. The strata vary from a few inches to a few feet in depth, and from the 

 variety of their colours, and from the singular forms and grouping of the hills 

 into which the great Bluff of the high Prairie is gullied by the torrents of ages, 



