GEOLOGY. 221 



that, as the level of the valley of the Missouri gradually rises, 

 there is a corresponding depression in that of the formation. 

 But what remains of it at the spot where I left it, conjoined to 

 information gathered, leads me to suppose that other traces 

 of it will be found, perhaps, not far from the Yellow Stone 

 River ; so that its whole extent along the Missouri, in a 

 generally NW. course, would be no less than 1,000 miles. 

 As to its western limits, without pretending to define these 

 positively, I may state that I have in my possession interest- 

 ing fragments of ammonite placenta and baculite ovatus, 

 brought to me from the clay-banks of the upper part of 

 Shayen and White Rivers. It is, therefore, probable that 

 the extent of the formation, due W., is not less than 250 miles 

 by water, along which it is, probably, open to examination. 



" The plateau of the Coteau des Prairies is composed, in 

 a great measure, of the materials belonging to what I have 

 named the erratic deposit, as is evidenced by the nature of 

 its soil, the physiognomy of the ridges and hillocks that 

 diversify its surface, the deep ravines by which it is flanked, 

 and the innumerable erratic blocks strewed over the borders 

 of its lakes. 



" We have no data by which to determine the inferior 

 limits of this deposit ; still, there is reason to think it rests 

 upon such primary rocks as show themselves along the line 

 of rapids of the Upper St. Peter's, consisting of granite, 

 sienitic, and other metamorphic rocks. Nevertheless, over 

 the vast extent of this plateau, there is, apparently, but one 

 spot where the subjacent rock makes its appearance — and 

 this is at the Indian Red Pipestone Quarry, so called. 



" The Falls of St. Anthony form the limits of the calca- 

 reous deposit that characterizes the shores of the Missi- 

 sippi from the mouth of the Wisconsin. The rocky forma- 

 tions then assume another type, being the several varieties 



