246 Geology and Botany of Northern Pacific Railroad. 



tains, the rock substructure is visible ; but the intervening 

 plateaus are covered with a sheet of drift clay and boulders, 

 that varies greatly in thickness, as it is spread over a rock-sur- 

 face that was once deeply and irregularly eroded. For example, 

 near the Upper Falls of the Missouri, where the banks of the 

 river are perhaps a hundred feet high, of solid rock, a tributary 

 coming in from the south cuts across an old valley filled with 

 drift which extends almost to the present river channel. At ils 

 mouth, this tributary has high rocky banks ; but a few hundred 

 yards above, they are altogether composed of drift. This drift 

 is a true till, thickly set with boulders,, some of which are two 

 feet or more in diameter. They are usually rounded, sometimes 

 subangular, and are composed of gray or red granite, quartzite, 

 palaeozoic limestone, and a variety of eruptive rocks. The re- 

 semblance of this drift to that from the Canadian highlands, is 

 so great that I was only convinced of its local origin when I 

 found all of its constituents in place in the Belt and Rocky 

 Mountains. The granites were to my eye indistinguishable from 

 those of the eastern Laurentian series ; they are of Archaean age, 

 as I subsequently learned ; and nothing but careful microscopic 

 examination will show them to be distinguishable, if they are so. 

 These facts lead me to suspect that even the very careful and 

 experienced observers who have reported the finding of eastern 

 Laurentian boulders on the flanks of the Rocky Mountains, 4,000 

 feet above the sea, may have been misled by this striking resem- 

 blance. 



On the undulating surface of the table-lands between the tribu- 

 taries of the Missouri, large boulders are occasionally seen, as in 

 the States bordering the Great Lakes ; and one of these, some- 

 what angular in form, has served so long as a rubbing-post for 

 the buffaloes which recently abounded in that region, that its 

 sides are all polished and a deep furrow is worn around it. 



Immediately south of the Falls of the Missouri, an extensive 

 coal-basin of Cretaceous (?) age is opened by the valleys of the 

 streams which come dow T n from the Belt and Highwood Mount- 

 ains. Two coal seams are exposed, one thin, the other from 12 

 to 18 feet in thickness, the latter a compound seam, some of the 

 benches of which are bright, pure coking coal. 



The Falls of the Missouri, caused by beds of sandstones belong- 



