8 SURFACE GEOLOGY. 



tinuous in the direction of the valley. They seem to have constituted shores 

 when the waters were higher, when less land was above the surface, and conse- 

 quently the waves had less power to wear away and comminute the rocks. 



6. Passing beyond and above the terraces and beaches, thus lying at the bottom, 

 and along the sides of the valleys, we reach the genuine drift deposit (F, Fig. 1, 

 Plate XI,) consisting of materials that are coarser, more angular, and less arranged 

 in strata and laminae. These are strewn promiscuously over the hills, except those 

 quite steep and high. They are also seen occasionally in the vallej^s, wherever the 

 terraces and beaches have been worn away or never existed. Yet it must be con- 

 fessed that it is often not possible to draw a distinction between the oldest beaches 

 and the drift. They pass insensibly the one into the other. The large blocks of 

 the drift are indeed frequently angular, but they are mixed with finer materials 

 that have been ground down and rounded, either by aqueous or glacial agency; 

 and the oldest beaches seem to be of essentially the same materials, somewhat 

 more modified. 



It is important, also, to mention that what appears to be genuine drift, is some- 

 times found mixed with, and sometimes superimposed upon, the beach and terrace 

 materials. This is especially true of large erratic blocks. And it shows us that 

 the drift agency, whatever it was, occurred in some places, after the modifying 

 agency that formed the older beaches and terraces had been for a time in operation. 

 Or, more probably, it was the same agency in modified forms that produced all the 

 phenomena. Below the drift we find the consolidated strata. (G, Fig. 1, Plate XL) 



The views that have now been presented I have attempted to exhibit to the 

 eye on Fig. 1, which is an ideal section across a valley, showing the manner in 

 which the terraces, beaches, and drift are usually found ; the newer deposits being 

 chiefly formed by the denudation and modification of the drift which lies beneath 

 the others. But as to the number of terraces, their relative height, &c, we find in 

 nature a great variety, and this section is intended only to give the general impres- 

 sion that has been made on my mind by all the cases which I have examined. 



Origin of the Materials. 



1. I have already said that the beaches and terraces appear to be mainly modi- 

 fied drift. The agency by which the former have been produced, commenced the 

 process of separation and comminution, carrying it at first only far enough to form 

 the higher and coarser beaches. The work still went on with another portion, till 

 it was reduced into finer materials for the higher terraces — and still finer for the 

 lower terraces, until, when it came to the lowest of all — our present alluvial mea- 

 dows — the fragments had been brought into almost impalpable powder, so as to 

 form fine loam. 



2. Such a work could not go forward with fragments already detached from the 

 ledges, as was drift, without subjecting the solid rocks to erosion, wherever 

 exposed. Accordingly a part of the materials of the terraces and beaches must 

 have been derived from this source. How deep in any place these erosions have 



