XIV INTRODUCTIOX, 



tlie sand been dug away that but little of the original surface of the 

 ground has been left. The outlier at Ilighgate spreads from a little 

 north of '' The Wrestlers " southwards to the higher part of the 

 cemetery, and then eastwards a little beyond the Archway. Besides 

 these outliers there may be some other small patches of the Bagshot 

 sand, as on the top of Parliament Hill, between Hampstead Heath and 

 Kentish Town ' (pp. 61, 62). Coarse iron sandstone occurs in the sand 

 at Plarrow and Highgate. 



Post-Fliocene. — Southern Boulder Clay.— According to Mr. Prest- 

 wich, Boulder clay occurs on the Finchley Hills {R. Geol. Soc. vol. xii. 

 p. 133). It has also been met with on Muswell Hill. 



High-Level Gravel. — At Stanmore Hill a drift of pebbles (rounded 

 flint-s) in sand and loam spreads over the whole of the hill-top. At 

 Potter's Bar there is also gravel. At Hendon there is pebble gravel 

 on the hill-top, and some of a fine nature is found in places on the 

 Bagshot outlier at Hampstead (p. 69). 



At Hadley and Barnet, and thence along the ridge westward, the 

 London clay is capped vdih gravel containing angular flints (p. 70). 



Between Muswell Hill and Finchley Common a gravel deposit 

 exists. It is about three-quarters of a mile wide, about 150 yards 

 broad, and from 15 to 20 feet deep. Immediately beneath the 

 vegetable soil is a bed about 14 feet thick, containing water-worn 

 fragments of granite, porphyry, micaceous sandstone, mountain 

 limestone, coal, lias, oolite, and chalk, with many of the characteristic 

 fossils of these formations. The most abundant are lias and chalk, 

 the latter sufficiently so as to give the whole accumulation a chalky 

 character. Beneath it is a bed, about 6 feet thick, of red pebble 

 gravel resting on London clay (pp. 70, 71). The uppermost of these 

 gravels is composed of the boulders of the southern boulder clay, the 

 cementing materials having been separated from the heavier portions 

 by running water, to form the accumulations of brick earth which are 

 occasionally found on the higher ground. ' A pebbly brick earth is 

 found on the London clay in many places : thus, near Bentley, west 

 of Stanmore, there is brick earth twelve feet thick, not so sandy as 

 that of the Thames valley, and which burns to a red brick.' ( Whitaker, 

 p. 68.) 



The Eev. J. C. Clutterbuck remarks, that the porous character of the 

 Lower Bagshot beds, and Higher Level Gravels which cap, more or less, 

 all the higher elevations, permit the percolation of water, which, being 

 upheld by the subjacent impervious clay, is thrown out in springs; 

 amon^.st which are the small perennial sources of the Eiver Brent, 



