146 



of the strata. — Four such periods are enumerated as having left dis- 

 tinct traces in the English strata. 1 . That which has formed the 

 pudding-stone of the old-red-sandstone, ascribed to the elevation of the 

 transition rocks. 2. That which has formed the conglomerates of the 

 new-red-sandstone, ascribed to the elevation of the carboniferous rocks. 

 3. That which has formed the gravel beds of the plastic clay. 4. That 

 which has produced the superficial gravel, spread alike over the most 

 recent and oldest rocks as a general covering, and which is found to 

 contain bones of extinct mammalia : this (it is agreed) may be iden- 

 tified as the product of one agra, by the same evidence which is em- 

 ployed to demonstrate the unity of any other geological formation. 

 Although diluvialists have usually directed their principal attention 

 to the effects of the currents of this latest epoch of general disturb- 

 ance, they by no means exclude the co-operation of any of the causes 

 above enumerated. 



In the body of his paper, the author considers the physical history 

 of the Thames as divisible into the following sections. I. The col- 

 lection of its head waters from the drainage of the Cotteswold uplands. 



II. The passage which it has forced across the Oxford chain of hills. 



III. That opened in like manner across the Chiltern hills to the 

 London basin at Reading. IV. There-entry of the river among those 

 hills by the Henley defile. V. Its course through the plains of London 

 to the sea. 



I. The head-waters of the Thames are collected from the drainage 

 of the Cotteswold uplands, over a tract about 50 miles in length, 

 constituting the rivers Isis, Churn, Colne, Lech, Windrush, Evenlode, 

 and Cherwell ; this chain of hills being entirely broken through by 

 the Colne, Evenlode, and Cherwell, which rise from sources in the 

 Lias plains beyond its escarpment. The height of most of these 

 sources is calculated at about 400 feet above the sea. 



Each of these valleys is separately described, and the general fea- 

 tures of denudation presented by the Cotteswold chain are pointed 

 out 5 these, it is asserted, bear traces of the most violent action, and 

 they are contrasted with the state of repose which has evidently pre- 

 vailed in the same districts from the period to which our earliest 

 historical monuments ascend. In the most exposed situations, and 

 those which appear to have suffered most from the action of the de- 

 nuding causes, earth works of British and Roman antiquity are fre- 

 quently found, attesting by their perfect preservation that the form 

 of the surface has remained unaltered since the time of their con- 

 struction. The drainage of the atmospherical waters has here produced 

 no sensible effect for more than fifteen centuries : it is inferred, 

 therefore, that to assign to this cause the excavation of the adjoining 

 valleys, 600 or 700 feet deep, is to ascribe to it an agency for which 

 we have no evidence ; the evidence, indeed, as far as it can be ex- 

 amined, being adverse. 



The disposition of the water-worn debris drifted against the Cot- 

 teswold chain and through the breaches opened in it, is also examined; 

 and much of it is shown to be derived from rocks situated to the north 

 of the valley of the Warwickshire Avon, and to be completely cut off 



