160 CHRONOLOGY OF FLUVIATILE DEPOSITS. CHAP. IX. 



depth from the surface, and yet be referable each to two sub- 

 divisions of the post-pliocene epoch, separated by thousands 

 of years. 



The relation of the glacial period to alluvial deposits, such as 

 that of Gray's Thurrock, whei-e the Cyrena Jiuminalis, Unio 

 littoralis, and the hippopotamus seem rather to imply a warmer 

 climate, has been a matter of long and animated discussion. 

 Patches of the northern drift, at elevations of about two 

 hundred feet above the Thames, occur in the i^jeighborhood 

 of London, as at Muswell Hill, near Highgate. In this drift, 

 blocks of granite, syenite, greenstone, coal-measure sandstone 

 with its fossils, and other palaeozoic rocks, and the wreck of 

 chalk and oolite, occur confusedly mixed together. The same 

 glacial formation is also found capping some of the Essex hills 

 farther to the east, and extending some way down their 

 southern slopes towards the valle}^ of the Thames. Although 

 no fragments washed out of these older and upland drifts 

 have been found in the gravel of the Thames containing 

 elephants' bones, it is fair to presume that the glacial formation 

 is the older of the two, for reasons given before at p. 130, 

 and that it originated, as we shall see in a future chapter, when 

 the greater part of England was submerged beneath the sea. 

 In short, we must suppose that the basin of the Thames and 

 all its fluviatile deposits are post-glacial, in the modified sense 

 of that term; i.e. that they were subsequent to the marine 

 drift of the central and northern counties, and to the period 

 of its emergence above the level of the sea. 



Having offered these general remarks on the alluvium of 

 the Thames, I may now say something of the implements 

 hitherto discovered in it. In the British Museum there is a 

 flint weapon of the spear-headed form, such as is represented in 

 fig. 8, p. 114, which we are told was found with an elephant's 

 tooth at Black Mary's, near Gray's Inn Lane, London. In 

 a letter dated 1715, jirinted in Ilearne's edition of ''Leland's 



