234 



THE WONDERS OF GEOLOGY. Lect. III. 



The boulders and masses of sandstone, which are abun- 

 dant in some of the chalk valleys, and on the flanks of the 

 Downs, and are called Sarsden-stone, or Druid sandstone, 

 from being the principal material employed in the con- 

 struction of Stonehenge, and other Druidical monuments, 

 have been derived from the lower Bagshot sands. In many 

 of the wastes and unproductive heaths around London, 

 the arenaceous deposits of this group form the subsoil ; and 

 they constitute that picturesque spot, Hampstead Heath, and 

 cap Highgate Hill and the adjacent heights. The beds of 

 shingle associated with the sands, have unquestionably been 

 derived from the ruins of the chalk formation.* 



23. Nature of Springs. — From the alternation of 

 sands and other porous strata with stiff impervious clays, 

 throughout the London basin, the metropolitan district is 

 favourable for obtaining water by means of the borings 

 termed Artesian wells, by which means perennial fountains 

 are made to rise from the natural reservoir in the lowermost 

 beds of sand. The nature of these wells is easily explained. 

 The descent of moisture from the atmosphere upon the earth, 

 and its escape into the basin of the ocean by the agency of 

 streams and rivers, were noticed in the first lecture. The 

 rain falling on a gravelly or porous soil, descends through 

 it, till its progress is arrested by a clayey or impervious 

 stratum, which thus forms a natural tank that receives the 

 water, and a subterranean reservoir or pool will be the result, 



* Mr. Prestwich, whose indefatigable and able researches have 

 largely contributed to establish correct views of the natural groups 

 of which the British tertiary consist, regards the Bagshot sands as a 

 triple series ; arenaceous strata constituting the upper and lower, and 

 argillaceous beds the middle division : to this last he refers the blue 

 clay of Bracklesham, Southampton, Barton, &c. However this maybe, 

 the geological character of the eocene system of the London and Hamp- 

 shire basins, is that of an extensive marine formation, with a few 

 intercalations of fluviatile deposits in the lower groups, and passing 

 into a freshwater and lacustrine condition in the upper. . 



