Notes upon the Diluvial and Alluvial Deposits of the Taffe Valley. 277 



partly from their grey and weathered aspect, and partly from their decom- 

 posed state, the structure of many of them being so loose, that a few rolls 

 would break tiiem up, a state to which they must have been reduced since 

 their arrival in their present place. 



The age of these stones, and the very slight quantity of debris (in fact 

 scarcely any at all), which has been formed since by far the greater number 

 of them were placed where they now lie, render it probable that when the 

 mountains had worn down to their present form, they ceased to weather with 

 any thing like the same degree of rapidity, just as any tolerably soft acute 

 hill would speedily become an obtuse one, and then undergo very little fur- 

 ther alteration of figure. 



III. This brings us to the gravel beds formed by the present rivers. It 

 has been already observed, that the diluvial gravel exhibits marks of a con- 

 siderable stream, in the form of channels and intersected banks. It is 

 within these larger channels that the present river is found to run, occu- 

 pying a channel much narrower than the old one, and hollowed out from 

 twenty or twenty-five feet deep in its bed. Besides this, but still within the 

 great channel, are occasionally the marks of old channels formed by the 

 present river, but which it has now deserted. 



The banks of the present river are, of course, in all such cases, sections 

 of gravel beds (except in some few instances where the river approaches 

 very near the base of a mountain, and makes a section of the debris), and 

 the pebbles of the bed of the river are for the most part derived from these 

 banks, and are therefore ready rolled. 



These modern gravel beds form knolls and tongues, and present, though 

 on a much smaller scale, all the phaenomena of the larger beds. The ob- 

 servations made above, with respect to the diluvial gravel beds, are appli- 

 cable also to these. 



IV. The alluvial deposits found in and below the Taflfe valley are of two 

 kinds j one, deposited by the Taflfe itself, or some of its tributaries j and 

 the other due exclusively to the waters of that arm of the sea into which 

 they fall. 



1. The gravel beds and debris of the old or larger channel, which is in 

 some parts, as below Newbridge, of very great breadth, are in many places 

 completely covered up by a stratum of sand or silt, of from one to four feet 

 thick, upon which the present vegetation grows, and which has not there- 

 fore for a very long period been much, if at all, added to. This deposition 

 appears due to the overflowings of the present river ; not indeed that its 

 waters ever flow within several feet of that level, at present, even in the 

 highest flood ; but it is presumed that before the country was inhabited, 

 the fall of trees and brushwood, and the want of proper attention to the 

 state of the banks and drainage of the soil, must have rendered very high 

 floods a much more common occurrence than they now are. 



2. The deposition at the embouchure of the Taffe is mud, and not 

 No. 5.— Vol. I. 2 o 



