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FORMATIONS OF THE INTERIOR 



Drift of the Prairies of Iowa. — On the west side of the Mississippi, in the vast 

 prairie region of Iowa, the attention of the geologist is frequently arrested by erratic 

 blocks, of enormous dimensions, scattered here and there, and half sunk in the 

 ground. Unlike the boulders we have just been considering, they are far from their 

 original situation. As they rise amid the ocean of grass, they may be seen for 

 miles ; and, in the absence of more conspicuous objects, they form the principal 

 landmarks of the traveller. The largest of them might, in an inhabited country, 

 very well be mistaken for cabins, in the distance. The one here represented was 



BOULDERS OF PORPHYRITIC GRANITE, IOWA. 



measured, and found to be fifty feet in circumference, and twelve feet high. It is 

 probable that at least one-half the rock is buried beneath the ground. Hence 

 may be gathered some idea of their huge dimensions. 



These boulders appear to be most abundant along the route which I travelled, 

 between the head waters of the Wapsipinicon and Red Cedar, and some ten to 

 fifteen miles beyond the latter, along a belt which may be twenty to thirty miles in 

 breadth. 



Among the smaller of these erratic blocks there is considerable variety ; it is, 

 however, somewhat remarkable, that almost every large boulder which I examined 

 in this region is a peculiar variety of porphyritic granite, in which the felspar is 

 of a flesh-colour, and often in large, regular crystals. Of the granite which I found 

 in place, in the interior of the Chippewa Land District, along my route to Lake 

 Superior, that which was found at the first rapids of the Court Oreille River 

 comes nearest to the composition and appearance of these prairie boulders. This, 

 however, can hardly be the source whence they have drifted ; for the direction of 

 the belt of erratics does not appear to be transverse to the streams, that is, from 

 northeast to southwest, but rather parallel with them, from northwest to southeast. 



The only explanation that is at all satisfactory in accounting for the transporting 



