88 ON EROSIONS OF THE EARTH'S SURFACE. 



for the top of that mountain shows as distinct marks of erosion as any other por- 

 tion of the valley. 



Under the quaint term of Purgatories, I introduce another evidence of oceanic de- 

 nudation, which we can connect with operations now going on in some places upon 

 the coast. In several works on geology I have given examples of this peculiar sort 

 of erosion ; yet they do not seem to have arrested the attention of geologists, at 

 least under this name. Of the origin of the name I know nothing, but I take it as 

 I find it among the people. Along the coast we find sometimes "long and deep 

 chasms in the rocks, into which the waves still rush during storms, and by their 

 concussion wear away the strata and lengthen the gorge. Sometimes this work 

 is aided by the jointed structure of the rock, which produces parallel fissures, and 

 when these are only a few feet apart, they enable the waves to carry on the work 

 of erosion more rapidly. In my Report on the Geology of Massachusetts, I have 

 pointed out two striking examples of these Purgatories, where we see the process 

 •actually going on, on the southern shores of Rhode Island. It is no wonder that 

 those who never thought of the manner in which they were produced, should have 

 given the same name to a much larger and longer chasm in the town of Sutton, 

 Mass., far removed from the ocean. I have discovered another at a still greater 

 altitude and further from the ocean, in Great Barrington, Mass. But I have given 

 so full an account of these cases in my report above referred to, that I need only 

 refer to that work. I can imagine no other explanation of their origin that will 

 at all meet the facts, although this has its difficulties. 



I will here refer to another example, which, .although I have not seen it, I 

 have been led from Dr. Charles Jackson's description to regard as a Purgatory. I 

 refer to the Dixville Notch, in the north part of New Hampshire. Here, through 

 the summit of a mountain of mica slate, which forms the dividing ridge between 

 the Connecticut and Androscoggin rivers, we find a fissure from 600 to 800 feet 

 deep, with nearly perpendicular sides. Its situation forbids the supposition that 

 the gulf can have been produced by fluviatile action, since the streams here run 

 in opposite directions into the Connecticut and Androscoggin. But it is just 

 the situation where the waves of an increasing or retiring ocean would act most 

 powerfully. The chasm may have been once occupied by a trap dyke, as supposed 

 by Prof. Hubbard (American Journal of Science, vol. ix. p. 160, N. S.). But my 

 inquiry simply is, how the trap, or other material which once filled it, has been 

 removed. And it seems to me that we must resort to oceanic agency. 



I have little doubt that careful examination would discover many more of these 

 "Purgatories" in the mountainous parts of New England. Indeed, what are the 

 famous "Notches" at Franconia and the White Mountains, but examples somewhat 

 modified of the same kind ? 



3. Erosions by Drift. 



This agency, as I have endeavored to show in another place, may be regarded 

 as chiefly or entirely ice and water. Yet these causes have operated under such 

 circumstances as to demand a notice distinct from that of the ocean, or of glaciers. 



