144 FORMATIONS OF THE INTERIOR 
Drift of the Prairies of Jowa.—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, LOWA. 
. 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 
