136 Artesian Well Boring in Southeastern, Minnesota — Jlatt. 



are thinner. At Brownsville granite is struck at 590 feet, at La 

 Crosse at 550 feet, and on the high prairie at Mason City the 

 granitic rocks are 1,475 feet below the surface. So it would ap- 

 ]»ear that there is but little room for the Potsdam in this section 

 between the white sandstone of the lower Saint Croix and the 

 granitic floor beneath the Cambrian of the Northwest. It is quite 

 likely that during the early part of the period of sedimentation 

 which followed the forming of the Lake Superior trough, the high- 

 land of northern central Wisconsin extended down to and across 

 the Mississippi river at La Crosse, and that the shore debris did not 

 cover it until the Saint Croix deposits of white sand were formed. 



15. Below these red sandstones and shales lie the crystalline 

 rocks which have in the northwestern states an enormous develop- 

 ment. It is not necessary here to enumerate their sub-divisions, 

 nor the rock species representing them. We will designate them 

 simply as Pre-Cambrian. They are permeated with water through- 

 out, but it percolates so slowly that no supply can be secured from 

 that, which ordinarily seeps through the rocks. Exceptionally^ 

 wells penetrating these rocks furnish a water supply, but it is 

 when fissures are struck through which the water runs in 

 streams. 



It will be seen, if now we glance back, that the formations 

 numbered 1, 5, 7,9, 11, 13 and 14 are water-bearing. For ordinary 

 well supply number one is universally used; for deep wells numbers 

 five, nine and eleven are the most important sources of supply, 

 because of their texture, which permits them to hold large quan- 

 tities of water reservoired, and allows it to seep through them 

 with great raf)idity. Judging from experience, a well borer who 

 reaches either one of these layers will never fail to secure a perma- 

 nent supply of water, while his supply will very likely be increased 

 if he goes beyond the layers named and into those below them. 



The quality of this water supply is high. The great depth 

 from the surface from which it is drawn prevents contamination 

 from organic impurities and insures constant uniformity in com - 

 position. It is not pure water — pure water is an unknown thing 

 in nature — but it is wholesome and at all times safe. The doctor 

 has just told us of the composition of these waters as compared in 

 bulk with those of surface wells, page 127 ; he has given the analyses 

 of a great number of wells to show what artesian water is, as a rule. 

 Let me give two analyses from well known wells, that we may see 



