GEOLOGY. 327 



exhibited by the rocks of Vermont. A satisfactory proof of the existence of 

 currents of water in particular localities in former times, was to be found in the 

 " pot-holes " in solid rock ; and numerous examples of these in Termont, 

 at various elevations above existing streams, showed that the whole valley 

 had been worn out beneath the pot-holes. In New-Fane, they were found 

 300 feet above the valley. In North Wardsboro', GOO feet; and in West 

 Wardsboro', upon the dividing ridge between two valleys, 1200 feet above 

 the valley, and nearly 2000 feet above the ocean. In fact, these pot-holes 

 may indicate the courses of large rivers upon this continent in former peri- 

 ods, when the configuration of the continent was much different from what 

 it is at present. 



Another proof of erosion was to be found in the varied inclinations of 

 strata. If the same kind of rock was found in the places dipping in oppo- 

 site directions, like the roof of a house, it was reasonable to infer that they 

 were formerly connected together, and by prolonging upon the paper the 

 inclinations and distances of these strata, the amount of rock that had been 

 worn away could be measured exactty. Sections now show where this 

 measurement might be made, and it was stated that in some cases it might 

 be clearly seen that at least four miles in thickness of rock had been removed 

 by erosion. 



Another and new proof of the amount of erosion was suggested by the 

 position of lofty peaks of protrusive granites, etc. Mount Ascutney, in Ver- 

 mont, was instanced as an example of granite extending more than two 

 thousand feet above all adjacent rocks. When the granite was melted, it 

 must have been sustained in its place by other rocks that were sedimentary 

 and not igneous ; otherwise the melted granite would have flowed over the 

 surrounding rocks. Hence, at this locality, as much as 2000 feet of mate- 

 rial must have been eroded, leaving the granite mountain now towering fax- 

 above the adjacent ledges. 



ORIGIN OF THE PRAIRIES OF THE NORTH-WESTERN UNITED STATES. 



In the Geology of Iowa, Vol. I. by Hall and Whitney, the various theories 

 which have, at different times, been brought forward to account for the ab- 

 sence of trees in the prairie region, are discussed and pronounced to be inad- 

 equate. It is attempted in the report to show that the extreme fineness of 

 the particles of which the soil is made up, is the predominating cause of this 

 peculiar condition of the vegetation, and some facts are stated to confirm 

 this theory. Reasoning from analogy of the smaller prairies to the thickly- 

 wooded region of the Upper Peninsula of Michigan, it is inferred, " that the 

 whole region now occupied by the prairies of the North-west was once an 

 immense lake, in whose basin sediment of an almost impalpable fineness 

 gradually accumulated, under conditions, the discussion of which is post- 

 poned to another volume, in which the drift phenomena of the North-west 

 will be taken up ; that this basin was drained by the elevation of the whole 

 region, but, at first, so slowly, that the finer particles of the superficial de- 

 posits were not washed away, but allowed to remain where they were origi- 

 nally deposited. After the more elevated portion of the former prairies had 

 been laid bare, the drainage becoming concentrated in narrower channels, 

 the current thus produced, aided perhaps by a more rapid rise of the region, 

 acquired sufficient velocity to wear down through the finer material on the 

 surface, wash away a portion of it altogether, and mix the rest so effectually 



