156 



NATURAL SCIENCE. 



Sept., 



sand, or mud {c'.c'.). Similarly, if there be slanting cracks, enclosing 

 between them a wedge-shaped portion of rock (a), then with each 

 contraction the wedge sinks, perhaps only a micromillimetre ; but it 

 sinks and sinks continually, and with each expansion the same lateral 

 pressure forces the rock-masses in a horizontal direction. 



But this system of forces can only act to a certain distance. 

 Various theoretical considerations, such as the limit to which surface 

 variations of temperature extend, and the resistance of the igneous 

 rocks to a breaking strain, led Nordenskiold to the conclusion that 

 the vertical cracks could not penetrate further than from thirty to 

 forty metres below sea-level ; and that, at about that depth, there 

 would be formed a series of horizontal cracks (b.b.), separating the 

 upper moving portions of rock from the more deep-seated lower 

 portion. If these conclusions were correct, it would follow that the 

 water which slowly soaked down the vertical cracks would collect in 

 and flow along the deep-lying horizontal cracks, and that there would 

 be a large and constant source of fresh water at thirty to forty metres 



-b-^m 



Diagram explaining Origin of Deep-seated Water. 

 sketch by Baron Nordenskiold. 



Facsimile of a rough 



below sea-level. With the aid of the diamond drill, this conclusion 

 has lately been put to the test, and so far as the constant pressure of 

 fresh water at the indicated depth can confirm it, the theory is fully 

 confirmed. 



The scientific consequences of this theory are evident and 

 important. The recognition of large quantities of water included in 

 and passing through the solid masses of gneiss, granite, and the like, 

 is welcomed by those who ascribe to aqueous agencies the secondary 

 changes that they believe to be still taking place in such rocks. 

 Here, too, if one will .but grant sufficient time, is suggested a potent 

 cause of lateral pressure, of cleavage, and even of overthrusts and 

 shearing. 



But the confirmation of the theory has consequences no less 

 important from the practical point of view. An entirely new water- 

 supply is made available, and one can incur the considerable expense 

 of the boring, confident that the money will not be wasted. Such a 

 source of fresh water is of special value to lighthouse keepers on 



