64 Geological Society. 



flowed off in another direction. There is, however, another alterna- 

 tive 5 and the interval between these chains may be supposed to have 

 formed originally a uniformly inclined plane, from the summits of 

 the one to those of the other, along which the waters once flowed, 

 and which they have since furrowed (by perpetually deepening their 

 channels) into the present valleys. The author calculates the mass 

 of materials which must on this supposition have been excavated and 

 washed away, and contends that the drainage of atmospherical waters 

 along such an inclined plane (which would have a fall of 10 feet per 

 mile) does not afl'ord an agent adequate to such vast operations. 



The Oxford chain has suffered greatly from denudation, being 

 broken into several detached groups. 



Among these, some insulated summits are capped by patches of 

 gravel, partly derived from transition rocks, partly from the chalk 

 formation ; these prove the extent to which denudation must have 

 proceeded since they were lodged in their present situation, as they 

 must have been transported from their native habitats along uni- 

 formly inclined planes, which have subsequently been excavated. 



III. Issuing from the defile of the Oxford chain, the river flows 

 through the plain of Abingdon and Dorchester, being joined by the 

 Ock and the Thame. This plain, like that of Oxford, is deeply and 

 extensively covered with water-worn debris. It is also similarly bound- 

 ed by a lofty chain (like that of the Chilterns) on the south. An enor- 

 mous breach is opened in this barrier for the passage of the river ; 

 all the same arguments apply in this case which were previously 

 urged with regard to the passage of the Oxford ciiain. 



The Chilterns, like most other chalky districts, abound with dry 

 valleys, the rifted and absorbent structure of that rock not permit- 

 ting the rain waters to collect into streams : these valleys agree in 

 every other feature with those containing water courses, and have 

 been obviously excavated by the same denuding causes, which, in 

 this case, it is self-evident could not have been river waters. The 

 surface of the chalk has been deeply and violently eroded, and is 

 deeply covered with its own debris ; this action appears, in part, 

 to have taken place during the epoch of the plastic clay formation. 



IV. The river having passed this defile, enters for the first time 

 the London basin, near Reading j where it receives the Kennet, of 

 which the course is shortly described. It rises in the chalk marl, 

 beneath the chalk escarpment, a few miles beyond Marlborough ; 

 that escarpment being broken through in several places, to give pas- 

 sage to its head-waters. The author insists, again, on the contrast 

 between the extensive denudations which must have occurred in this 

 district and the permanence of its surface, as attested by the pre- 

 servation of the numerous Druidical and other British monuments 

 scattered over these downs. 



A little below Reading, the Thames (first having received another 

 small tributary, the Locidon) quits for a time the London basin, to 

 re-enter, by a sudden bend, another deep defile among the chalk 

 hills, ranging by Henley and Marlow to Maidenhead, when it finally 

 enters the plains of London. It is difficult to account for this de- 

 flection 



