62 Geological Society. 



of denudation, and therefore better suited to illustrate that operation 

 than valleys of more complicated origin, in the formation of which 

 the elevation and dislocation of the strata have co-operated. 



He first offers some introductory remarks on the opposite theories 

 of the fluvialist and diluvialist, the former ascribing such denudations 

 exclusively to the operation of the streams actually existing, or rather 

 to the drainage of the atmospherical waters falling on the districts, 

 which it is supposed have become thus deeply furrowed by the gra- 

 dual erosion of these waters, continued through a long and indefinite 

 series of ages ; the latter contending that such a cause is totally in- 

 adequate to the solution of the phaenomena, and maintaining that they 

 afford evidence of having been produced by violent diluvial currents. 



He proceeds to distinguish several different geological epochs, at 

 which it is probable that currents must have taken place calculated 

 to excavate and modify the existing surface. I. In the ocean, be- 

 neath which the strata were originally deposited. II. During the 

 retreat of that ocean. III. At the periods of more violent disturbance, 

 which are evidenced by the occurrence of fragmentarian rocks, the 

 result of violent agitations in the waters of the then existing ocean 

 propagated from the shocks attendant on the elevation and dislocation 

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

 tinct traces in the I^nglish strata. l.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 aera, by the same evidence which is em- 

 ployed to demonstrate the unity of any other geological formation. 

 Although diluvialisls have usually directed their principal attention 

 to the effects of the currents of t'nis latest epoch of general disturb- 

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

 above enumerated. 



In tlie 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 (^hiltern hills to the 

 London basin at Reading. IV. The re-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 tiicsc 

 sources is calculated at about 4U0 feet above the sea. 



Each 



