224 AMOUNT OF EROSION. 



PEOOF FEOM THE UNSTRATIFIED EOCKS. 



Another evidence of erosion, and a measure of it, is derived from the unstratified rocks. 

 Admitting, as most geolgists do, that they have been once in a melted state, it is obvious 

 that while in that condition they would flow into the lowest places, and could not be made 

 to stand up in walls or peaks, unless surrounded and supported by other rocks. If, 

 therefore, we find mountains cut through by dikes of trap or porphyry, or veins of syen- 

 ite and granite, we may be sure that other rocks must have surrounded and covered the 

 fissures as high as we now find them filled, otherwise the fused matter would have all run 

 out of the fissures. So if we find any of these rocks shooting up as naked ridges or 

 peaks, we know that when first melted or protruded they must have been buttressed up 

 by stratified deposits, until they were hardened. Nay, there is so much evidence that 

 granite was formed very deep in the earth and under powerful pressure, that, as a late able 

 geologist says, " we may feel sure that at the time of its consolidation, it was covered with 

 a thickness of at least several thousand feet of other rock, and that this thickness has 

 been removed by the gradual action of erosion by moving water." (Juke's Manual, p. 273.) 

 We must, therefore, add several thousand feet to the height above the present surface of 

 the most elevated ridges or peaks of granite, in order to ascertain the amount of erosion ; 

 and we ought to remember too, that the granite also has been cut down not a little, after 

 its cap rock had been abraded. 



With such rules for our guide, let us now turn to the unstratified rocks of Vermont, to 

 see what lesson they teach us. Generally the dikes of trap do not occur at a great height. 

 We think those at Mt. Holly, about 1400 feet above the ocean, or 1300 above Champlain, 

 are the highest, unless it be some small ones on Danby Mountain. But to prevent the 

 melted matter from flowing out of these, would require buttresses of stratified rocks nearly 

 as high as any part of the Green Mountains, and consequently high enough to fill up all 

 the present valleys of the State. 



Prof. 0. P. Hubbard, several years ago, presented an able paper in the American Jour- 

 nal of Science on trap dikes in New Hampshire, as an evidence and measure of erosion. 

 ( Vol. IX, N. Series, p. 158.) He there states that he found a trap dike cutting through 

 Mt. Pleasant, one of the White Mountain group, at the height of 4500 feet. This would 

 require a cap of stratified rock as high nearly as any part of the White Mountains ; and 

 hence Prof. Hubbard infers, with great reason, that "the deepest valleys" of New Hamp- 

 shire "are but valleys of erosion." 



We reach a similar conclusion by an examination of the granitic and syenitic peaks of 

 Vermont. We can hardly doubt that most of these have resulted from the metamor- 

 phosis of other rocks and have not been thrust up from the interior of the earth into cracks 

 and through the strata. But in all cases we cannot doubt that such metamorphoses would 

 bring the materials into a melted state, enough certainly to cause the mass to flow. In 

 some cases we have evidence of such a condition ; and since these embrace the highest 

 granitic peak in the State, we pass by all other examples and let this stand instar 

 omnium. 



Throughout the whole length of the Connecticut valley, we find occasional peaks of 

 granite and syenite which have indurated the adjoining rocks, and sent veins into them, 



