DRIFT AGENCIES. 67 



the waters left it. Such a fact corresponds better with the idea of a retiring 

 ocean than of a rising continent. And upon the whole, though I cannot doubt 

 that lateral pressure and internal volcanic force have produced limited vertical 

 movements ; I am more and more inclined to believe that the waters have in a 

 great measure withdrawn in the manner suggested by Prevost and Dana. 



40. The phenomena of drift, in distinction from terraces and beaches, although 

 an important part of surface geology, I have not dwelt upon in this paper, because 

 they are now generally known, and, so far as North America is concerned, I have 

 published them elsewhere. But some suggestions upon the theory of drift seem 

 important in this place, in order to bring out my views fully upon surface geology. 

 I have endeavored to show that a large proportion of what has been usually 

 regarded as drift, has been the result of subsequent alluvial agencies. There still, 

 however, remains an irregular coarse deposit" beneath the modified beaches and 

 terraces, whose origin is a matter of great interest. The subject is narrowed, but 

 not disposed of. There yet remain the great boulders, mixed with rounded frag- 

 ments and sand and clay, as well as the striated and embossed surfaces, to be 

 explained. And in respect to the agency by which the phenomena have been 

 produced, the following positions, which are most of them essentially those taken 

 by Professor Naumann, appear to me most unquestionably true : — 



1 . The eroding materials must have been comminuted stone. 



2. They must have been borne along under heavy pressure. 



3. The moving force must have operated slowly and with prodigious energy. 



4. It must have been nearly uniform in direction, yet capable of conforming 

 somewhat to an uneven surface, and of some divergence when meeting with 

 obstacles. 



5. The vehicle of the eroding materials cannot have been water alone. 



6. It must have been a firm and heavy mass, yet somewhat plastic. 



7. The grinding and crushing mass must have been impelled by such a vis a 

 tergo, as would urge it over hills of considerable height. 



8. A part of the phenomena can be explained only by the presence and agency 

 of water in some places, at least to sort out, arrange, and deposit layers of sand, 

 clay, and gravel, which are sometimes found beneath the large boulders that are 

 scattered over the surface, or sometimes mixed with the finer stratified deposits. 



Were this the proper place, I would quote a multitude of facts to sustain these 

 positions. But since to do this would be less original than the other parts of this 

 paper, I will refer only to a single observation, made by me in the White Moun- 

 tains, in 1851, and which I have described in the 14th volume, 2d Series, of the 

 American Journal of Science, p. 73, to illustrate the fifth of the above positions. 

 On the southwest side of La Fayette Mountain, near the Franconia Notch, I fol- 

 lowed the track of a recent summer slide, which had never been explored. The 

 perils which I encountered in this attempt, greater than I have ever met in a 

 mountain excursion, are detailed in the Journal of Science, but will here be 

 omitted, and I shall give only a part of the facts. 



I found a path several rods wide ploughed out by an immense mass of coarse 



