566 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



with salts of soda and magnesia, and there are few springs which rise 

 through it in which these substances are not dissolved. Where these 

 strata are exposed, they form barren tracts, whose surface is cracked 

 and crumbling, as though the access of moisture had occasioned a swell- 

 ing of the mass. Their consistence is soft, so that the limbs of horses 

 sink into them sometimes for their entire length, thus rendering it dif- 

 ficult to traverse them with pack-animals, or, indeed, impossible where 

 they constitute steep hills. Although to be found in other localities 

 and horizons, notably in the Wahsatch Tertiary in Wyoming, these 

 '' alkali beds " are nowhere so numerous or so extensive as in the region 

 of Cretaceous No. 4, on the Upper Missouri. The remains of Molluscs 

 and Vertebrates are chiefly confined to the soft sandstones, but they are 

 also abundant in the rusty-brown concretions which are scattered through 

 the shale. These round or oval bodies are fissured, and the cracks are 

 occupied with usually crystalline calcite. The dark shales are covered 

 by a layer of rusty sandstone of variable but not great thickness, which 

 generally contains many molluscous fossils. At the base of the shales is 

 another bed of sandstone of a lighter color, usually buff, and softer and 

 less laminated character. It readily wears into bad-land scenery, and 

 does not contain fossils in the few localities where 1 observed it. Below 

 tliis, the brown clay-slate reappeared at the only localities where I saw 

 the underlying bed, i. e., on Dog and Birch Greeks. The thickness of 

 the lower sandstone varies from 40 to 100 feet. Not far above it, in the 

 shale of No. 4, sometimes lying almost on it, is a bed of lignite, which 

 varies in thickness between 6 and 10 feet. In some places, for instance 

 on Dog Creek, this lignite possesses some value as fuel, burning with a 

 yellow flame, and giving out considerable heat, although it lights slowly. 

 In most other localities it is very impure, and contains dicotyledonous 

 leaves badly preserved. The characteristic shale which overlies this 

 bed is from 50 to 200 feet in thickness in the region of the Judith Eiver, 

 while on the lower river, for example in the Round Butte, near Fort 

 Peck, Mont., it exhibits a thickness of nearly 1,000 feet above the 

 water-level. Its layers are thin, and usually so soft as to yield readily to 

 the action of the weather; it is cut by drainage-ravines into rounded 

 hills, excepting where the overlying rusty sandstone protects it, when 

 it forms bluffs with steep naked sides. Important drainage-channels 

 cut deep caiions into it, and the Missouri engulfs huge masses whenever 

 its current impinges for a time against it. 



After passing through an uninterrupted body of this formation for 100 

 miles, the water of the river, at about 60 miles above Fort B uford , exchanges 

 its purity for the muddiness which gives it its name, and which it keei>s 

 to its mouth, even giving character to the Mississippi to its delta. The 

 river receives so much of this material from its banks that it is neces- 

 sarily depositing it at every point where the rate of the current is dimin- 

 ished ; hence the immense sand bars which everywhere obstruct its 

 navigation. From the same source is no doubt derived a large portion 

 of the sediment which forms the delta of the Mississippi. 



