CHAP. IX. POST-PLIOCENE ALLUVIUM OF ENGLAND. 155 



of fifty miles, having a width varying from two to nine miles. 

 Its thickness ranges commonly from five to fifteen feet.* In- 

 terstratified with this gravel, in many places, are beds of sand, 

 loam, and clay, the whole containing occasionally remains of 

 the mammoth and other extinct quadrupeds. Fine sections 

 have been exposed to view, at different periods, at Brentford 

 and Kew Bridge, others in London itself, and below it at 

 Ilford and Erith in Kent, on the right bank, and at Gray's 

 Thurrock in Essex, on the left bank. The united thickness 

 of the beds of sand, gravel, and loam amounts sometimes to 

 forty or even sixty feet. They are for the most part elevated 

 above, but in some cases they descend below, the present level 

 of the overflowed plain of the Thames. 



If the reader will refer to the section of the post-pliocene 

 sands and gravels of Meuchecourt, near Abbeville, given at 

 p. 122, he will perfectly understand the relations of the ancient 

 Thames alluvium to the modei'n channel and plain of the 

 river, and their relation, on the other hand, to the boundary 

 formations of older date, whether tertiary or cretaceous. 



So far as they are known, the fossil mollusca and mammalia 

 of the two districts also agree very closely, the Gyrena flumi- 

 nalis being common to both, and being the only extra- 

 European shell, this and all the speci-es of testacea being re- 

 cent. Of this agreement with the living fauna tliere is a fine 

 illustration in Essex; for the determination of which we are 

 indebted to the late Mr. John Brown, F.G.S., who collected 

 at Copford, in Essex, from a deposit containing bones of the 

 mammoth, a large bear (probabl}' Ursus spelceus), a beaver, 

 stag, and aurochs, no less than sixty-nine species of land and 

 fresh-water shells. Forty-eight of these were terrestrial, and 

 two of them. Helix incarnata and H. ruderata, no longer in- 

 habit the British Isles, but ai*e still living on the continent, 



* Prestwich, Geological Quarterly Journal, vol. xii. p. 131. 



