380 Geological Society, 



channels; into the present valleys. The author calculates the mass of ma- 

 terials 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 ft. per mile) does not afford 

 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 trans- 

 ported from their native habitats along uniformly 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 bounded by a lofty chain (like that 

 of the Chilterns) on the south. An enormous breach is opened in this bar- 

 rier 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 chain. 



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

 the rifted and absorbent structure of that rock not permitting 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 Lon- 

 don basin, near Reading, where it receives the Kennet, of which the course 

 is shortly described. It rises in the chalk marl, beneath the chalk escarp- 

 ment, a few miles beyond Marlborough ; that escarpment being broken 

 through in several places, to give passage 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 preservation 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 Loddon) 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 deflection of the river, as a straighter course 

 appears open to it by White Waltham to Bray. This line was surveyed for 

 a canal by Mr. Brindley, and appears to be level to White Waltham, and 

 thence to fall 47 feet to Mankey Island, near Bray ; so that a dam of a few 

 feet across the river below Sunning, at the mouth of the Loddon, would turn 

 the waters into this channel. The author conceives, the most natural mode 

 of explaining this deflection of the river is, by the supposition that a higher 

 range of tertiary strata once extended from the ridges of Bagshot heath in 

 this direction, forming a bar to the progress of the stream in this line. 



V. The plains of London are covered with enormous accumulations of 

 water- worn debris, chiefly of chalk-flints, and often abounding in fossil re- 

 mains of elephants, hippopotami, &c. ; the gravel is not confined to the low 

 grounds, but caps the highest summits of the district; e.g. Highgate on the 

 north, and Shooter's Hill on the south, of the river. To explain this distri- 

 bution of this gravel by the operation of the actual rivers, the author observes 

 that it is necessary, first, to suppose that a uniform plane originally existed 

 fi'om the summit of Highgate to the Hertfordshire chalk downs, and from 



