50 



Prof. J. Prestwich. 



Sperenberg bore-hole, the latter forms a mass several thousand feet 

 thick* 



The Influence of Water. — The above, however, are but local and 

 minor conditions, subordinate to one of a much greater and wider 

 influence. The conductivity experiments of Messrs. Herschel and 

 Lebour were, with few exceptions, made with blocks of dried rock. 

 In a few instances they repeated the experiment with wet blocks of 

 the same material, and with a remarkable difference in the result. 

 Thus— 



Conductivity. 



t A > 



Dry. Wet. 



New Red Sandstone .... '00250 '00600 



Quartzose sand '00105 '00820 



Clay '00250 '00350 



Mean '00202 '00590 



Here we have substances which when dry present great thermal 

 resistance, becoming when wet amongst the best of the rock conduc- 

 tors of heat — equal, if not superior, to that of the crystalline and 

 schistose rocks. 



This condition becomes, in considering the question of conductivity 

 in relation to underground temperatures, a matter of very great 

 importance, for in nature dry rocks are the exception and wet rocks the 

 rule. . The level of permanent saturation of the strata is regulated by the 

 sea level on the outside, and by the level of the river valleys and their 

 tributaries inland. All the rocks below those levels are, as a rule, per- 

 manently saturated with water, while between the valleys the line of 

 water level rises in proportion to the distance the water has to travel, 

 and the friction offered by the rocks before it escapes as springs. In 

 the chalk hills of Kent or Surrey, for example, which rise to the 

 height of 500 to 600 feet, the water level stands 200 to 300 feet beneath 

 the surface, while below that level, the rock, whatever its thickness, 

 is in a state of saturation. In the valleys the chalk is saturated to 

 the surface ; and Artesian Wells are always in relatively low levels. It 

 is the same with the sandstones of the Trias or of the Coal-measures, 

 only that in the latter the presence of faults often cuts off the supply, 

 and segments exist with but little water except that of imbibition. 

 This water of imbibition, or quarry water, is present in rocks above 

 the line of permanent saturation, it being a property that depends on 

 the capillarity of the rock, which is very strong in chalk and oolite, 

 while it is slight in quartzose grits and sands. There are therefore 

 few rocks in which the influence of water is not felt. 



* It may, however, be a question whether it is not intercalated with thin seams of 

 gypsum. 



