ORIGHN AND DEVELOPMENT OP ORE-DEPOSITS. 165 



Many other specific characters in the country rocks have 

 been from time to time regarded as unfavourable, such as a 

 harsh texture, and an absence of moisture, especially if porous. 



It will be readily seen from what has been stated above 

 that all these characters have a sound scientific basis, while 

 others not so generally recognized will be noted hereafter. 



Sec. 7. — Subterranean Circulation. 



It is a matter of common observation that, subject to smaU 

 changes such as are due to variations of rainfall, evaporation, or 

 drainage, the water standing in wells and artificial excavations, 

 or flowing from natural springs, maintains a pretty constant 

 level, which is known as the natural water level of that particular 

 spot ; or less accurately by some writers, the natural water- 

 plane. In elevated regions of porous rocks and intermittent rain- 

 fall, this water level is often far below the surface of the ground, 

 and very important results, economical as well as geological, 

 depend upon this fact, as will be seen hereafter. It may be 

 locally altered by pumping or tunnelling ; or equalized over 

 larger areas by opening subterranean communications; but subject 

 to minor or local variations it is practically invariable over 

 large areas and for long periods of time. 



For our present purpose we may divide the waters circulat- 

 ing within the earth's crust into what may be called surface 

 waters, which do not descend beneath the (natural or artificial) 

 water-level, and those which, descending below that level, are 

 again brought up by ascending currents. The former — the 

 phreatic waters of Daubree will in general have temperatures 

 differing but little from the mean temperature of the region in 

 which they occur,* the latter which may be called crenic or crenitic 

 after Sterry Hunt,f are the thermals of Daubree. As by hypo- 

 thesis they come from deep-seated regions they will in general 

 have higher temperatures than the phreatic-waters ; bearing, too, 

 some direct relation in ordinary cases to the depths from which 

 they have arisen. As the temperatures are higher, so the solid 



* Unless of course they happen to pass through or over locally heated rocks 

 in which case they may be mistaken for true thermals. 



t Mineral Physiography, p. 132 (from Kprjvq Gr. a fountain or spring.) 



