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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 chain. 



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 ; 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 j 

 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 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 de- 

 flection 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 remains 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 distribution of this gravel by the ope- 

 ration of the actual rivers, the author observes that it is necessary, 

 jirst, to suppose that an uniform plane originally existed from the 



