96 MATERIALS. 



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. The ideal section which we have 

 referred to shows also the superposition of the different stratified beds. First and oldest 

 are the strata of solid rock, which are unconformable to the alluvium. The drift under- 

 lies all the terraces, and the highest terraces run under the lower ones. Hence the para- 

 doxical fact that the highest deposits are older than the lowest. This section also shows 

 what is the common state of things in regard to the relative number of terraces upon the 

 two sides of a valley. Some have stated that the terraces on both sides are the same in 

 number, and at the same level. Here both the number and the height above the river 

 are different upon the two sides ; and this illustration corresponds to the general arrange- 

 ment in Vermont ; for this ideal section conveys the general impression made on the 

 mind by all the cases that .have been examined. 



River terraces are generally arranged in basins, which vary in length with the rate of 

 descent of the bottom of the valley. Where a river descends so slowly as to appear 

 slugggish and muddy, a basin may extend for many miles ; but in rapid streams the basins 

 are more frequent, and are often at different levels. At the ends of the basins the solid 

 rock generally forms the banks of the river and always approaches near the water. 



ORIGIN OF THE MATEEIALS. 



1. It has already been stated that beaches and terraces appear to be modified drift. 

 The agency by which the first was 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 mate- 

 rials for the higher terraces, and still finer for the lower terraces, until when it came to 

 the lowest of all our present alluvial meadows 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 been made, may be learned by ascer- 

 taining how near the bed of the stream we find drift striae. Reasoning in this way we 

 find upon the La Moille River, for instance, that at East Fairfax the rock has been scarcely 

 denuded since the drift period, as the striae are within five feet of the water's edge ; while 

 at Wolcott, on the same stream, pot-holes have been found fifty feet above the water's 

 level. In other places rock to the amount of 200 to 300 feet has been eroded. 



ARRANGEMENT OF THE MATERIALS. 



1. Stratification and Lamination. All these deposits are more or less stratified, and 

 most of the finer varieties are also laminated. The lamination is not unfrequently oblique 

 to the stratification. The former is frequently inclined as much as twenty degrees to the 

 horizon, the latter usually quite horizontal, though the strata or laminae of clay arc 

 sometimes plicated. 



