and Marlborough Raihcarji. 189 



There are two ways by which we may endeavour to explain the 

 difference between the present level of the watershed at Lydeway, and 

 what it must have once been ; by subaerial degradation, lowering the 

 ground by the action of the weather, rain, streams, &c. ; or by an 

 oscillation of level. 



To the first of these causes great importance has been attatched 

 by Mr. Jukes, who goes so far as to ascribe to it the whole excavation 

 of certain valleys in the south of Ireland, and to express his belief 

 that as a general law the lateral valleys (such as the Avon Valley), 

 first began to be formed when the surface waters ran off at a high 

 level from the dominant ridge, which once occupied the place of 

 the longitudinal valley (such as the Vale of Pewsey); and that both 

 lateral valley and longitudinal valley, are due to long continued 

 wasting by subaerial causes only.' 



Let us see how far facts will bear us out in the supposition that 

 the ground at Lydeway has been lowered 90 feet by subaerial 

 degradation. 



The flint gravel of Pewsey Common is 157 feet above the Avon 

 at Rushall. The Avon therefore since it flowed through a con- 

 necting gap at that level, has lowered its course 157 feet, while the 

 ground at Lydeway has wasted 90 feet in height. The Avon at 

 Rushall, a tolerably rapid stream, confined in its course within 

 narrow limits, and flowing over chalk to a certain extent soluble, 

 has been at work lowering the valley under favourable conditions. 

 At Lydeway on the contrary, the land is flat, the streams sluggish 

 and almost stagnant, and the Loess like drift with which the country 

 is covered, seems to point to a long period during which the land 

 has rather gained an accession of height from flood deposits, than 

 suffered from subaerial degradation. A lowering of 90 feet at the 

 latter place compared with 157 feet at Rushall, seems then far too 

 great a proportion to be admissible. 



We must accept the alternative then, that a very large part, if 



not all, the lowering of the ground at Lydeway has been caused by 



an oscillation of the land. This need excite no surprise when we 



reflect on the changes of level which the land must have undergone 



' Journal Geological Society, Vol. XVIII., page 400. 



