1178 A TREATISE ON METAMORPHISM. 



for instance, the lead and zinc district of southwestern Wisconsin, already 

 described. (See pp. 1144 et seq.) Another excellent illustration of very 

 loose and open ground above the level of ground water and tight ground 

 below it is furnished by the Monte Cristo district of Washington." Here 

 near the surface all the minor joints are open, circulation has been free, 

 and the larger ore deposits are found. At depth the joints are mostly tight, 

 only a few being sufficiently open to allow of much water circulation. 



But numerous and large openings may exist below the level of ground 

 water. In various kinds of rocks — such as sandstones, conglomerates, 

 amygdaloids, and tuffs — the openings are original, and may not have been 

 closed by cementation. Of course, the more recent the earth movements 

 the more numerous and larger are the openings. In some places the 

 descending waters are not saturated when they reach the level of ground 

 water, and solution continues for some distance below it. Furthermore, 

 the level of ground water varies under different circumstances. Where a 

 region is being uplifted the level of ground water, other things being equal, 

 is descending, and where a region is subsiding it is rising. As a result of 

 physiographic changes there may be alternate valley filling and valley 

 erosion. These changes affect the level of ground water. In Pleistocene 

 time there was an extensive period of valley filling instead of erosion. 

 Consequent on this the level of drainage, and therefore the level of ground 

 water, rose. Also there may be very considerable variations in the level 

 of ground water as a consequence of long-continued climatic changes, such, 

 for instance, as the alternating periods of humidity and aridity in the Cor- 

 dilleras of the West in connection with the Pleistocene. 6 To illustrate, at 

 the present time in the mining districts of New Mexico and Arizona the 

 level of ground water is far below the surface, but it can not be doubted 

 that during the humid epoch evidenced by the existence of Lake Bonne- 

 ville and Lake Lahontan the level of ground water was much higher. 

 Emmons gives a specific case in which the ground water was apparently 

 once nearer the surface than at present. According to him, at the Delamar 



<*Spurr, J. E., The ore deposits of Monte Cristo, Washington: Twenty-second Ann. Rept. U. S. 

 Geol. Survey, pt. 2, 1901, p. 847. 



» Gilbert, G. K., Lake Bonneville: Mon. V. S. Geol. Survey, vol. 1, 1890. Russell, I. C, 

 Geological history of Lake Lahontan, a Quaternary lake of northwestern Nevada: Mon. U. S. 

 Geol. Survey, vol. 11, 1885. 



