IT.] SPRINGS. 27 



which it deposits on standing, sufficiently attest the presence 

 of iron: it is in fact a Chalybeate'^ spring. This example 

 sufficiently illustrates the origin of mineral springs. The 

 saline and other substances which they contain, and on 

 which their peculiar properties depend, are generally 

 dissolved out of the rocks through which the water flows. 

 In the basin of the Thames, mineral springs are by no 

 means uncommon, some being chalybeate, others sul- 

 phureous, and others again saline. This is a subject, 

 however, which will be considered by and by. 



What has been said, with respect to the structure of 

 the high ground at Hampstead, applies equally to 

 that at Highgate, and at Harrow. In each of these 

 localities, an isolated patch of Bagshot sand rests upon 

 London clay. And, coming down to the lower ground on 

 which the greater part of the metropolis is seated, a very 

 similar condition of things is met with ; that is to say, a 

 layer of highly pervious material is spread over a rock well 

 nigh impervious. The porous material, however, instead of 

 being Bagshot sand, is here a bed of gravel, varying from 

 ten to twenty feet in thickness, and resting upon the 

 London clay. When the rain has soaked into this gravel, 

 it is held up by the clay below, and is thus preserved in a 

 great underground reservoir, which offers a never-failing 

 source of supply to the shallow wells which were formerly 

 sunk in great numbers throughout London. Here and 

 there, a little valley cuts down through the gravel to the un- 

 derlying clay; and, the natural reservoir being thus tapped, 

 a spring of water flows out at the junction. Such is the 

 origin of the springs which gave names to Clerkenwell, 



^ Chalybeate from xaAui^', chaliips, steel or hardened iron, a Greek 

 word derived from the people called Chalybes, who dwelt on the 

 suulhern shore of the BUck Sea, and %\'ere famous iron- workers. 



