240 SURFACE GEOLOGY. 



soon afterwards* It appears that in 1850 a powerful rain soaked the 

 debris in this valley so that it slid a considerable distance downwards, 

 and left deposits analogous to moraines, but did not materially engrave 

 the ledges. 



At the head of the slide we saw a mass of naked gneiss rock many rods wide, mostly 

 denuded of soil, and much of it also of several layers of the rock, which had slid down- 

 wards, and were strown along the sides of the ravine for at least two miles. This naked 

 surface, at its upper part, had a slope of about 38°. Lower down, however, it was 

 much less, for the most part, and at its termination the descent was slight. * * * 

 It was just such an example as I had long wished to find. An enormous mass of detri- 

 tus, probably from ten to twenty feet thick, and in some places two or three rods wide, 

 composed of irregular fragments of all sizes, from twenty feet in diameter down to 

 sand, had been driven forward over a rocky surface two miles long. What, now, was 

 the effect upon the rocky floor? Did it score and striate the floor, as was done by the 

 drift agency, as some suppose would be done by the crowding forward of detritus by 

 the power of water? I found it was not so. The rock in place was smoothed but not 

 striated, except in a few places, perhaps in the slightest manner. The fundamental 

 rock passed over is gneiss, but it is traversed occasionally by veins of granite, and 

 towards the upper part by dykes of trap several rods wide. They are such rocks as in 

 various places retain distinct markings of the drift action. The beds of detritus pro- 

 duced along this slide are so closely like those of glacial origin that we may call them 

 moraities. They are larger and more distinct than I have seen on any other slide. All 

 along the borders of the ravine are ridges of blocks, gravel, and sand, sometimes 

 twenty feet high, lying in as much confusion as is possible, and making it difficult and 

 even dangerous to go into or out of the ravine over the loose and crumbling ridges. 

 At the lower end of the slide is a large terminal moraine, by which the river has been 

 forced to seek a new channel. This terminal moraine is, in fact, double ; that is, an 

 old moraine lies in advance of that produced by the slide of 1850, the blocks of the 

 two being easily distinguished by the appearance of recent or more ancient erosion. 

 In short, the appearances along this gulf are almost precisely what they would be if a 

 glacier in one of the valleys of the Alps should melt away. And when examining it, I 

 had no doubt that the slide was produced by the advance of a mass of ice ; yet I no- 

 ticed that, in some places, the lateral moraine was driven in among the trees without 

 affecting them. In some places near the bed of the slide I noticed the stumps of trees, 

 perhaps six inches in diameter, that had been broken off by the descending mass. 



This valley was regarded by Agassiz as the source of that glacier. In 

 confirmation of it, boulders of Franconia breccia like the ledge compos- 

 ing Eagle cliff, have been found near J. McDonald's, in Franconia, at 



'Am. your. Science, ii, vol. 



