HYDROLOGY. 



143 



according to circumstances. (1) "Water descending from above may 

 be caught and carried out where the strata are cut across to the proper 

 depth; and (2) water that gained access to the sandstone at some dis- 

 tant and more elevated point may rise from below at places where the 

 confining stratum is removed. So that it is proper to include in this 

 group some that issue from ledges somewhat above or below the junc- 

 tion of the formations. These springs are similar in general character 

 to the last, but usually contain a more considerable percentage of the 

 several mineral ingredients, at least that portion of them that are de- 

 rived from the limestone, which still retains traces of many of the 

 salts that we have reason to suppose were incorporated with it when 

 it was formed beneath the ancient ocean. 



To this class belong most of the springs that issue from the rock in 

 the western half of the counties of Rock, Jefferson, Dodge and Fond 

 du Lao. Though differing among themselves, they are, as a class, of 

 most excellent quality, and should be substituted for the well water 

 of those regions, so far as practicable. 



A number of springs in the vicinity of Beaver Dam issue from 

 near the junction of the Trenton with the Galena limestone; and at 

 other points, streams issue from the latter formation, but they do not 

 seem to be confined to any single stratum, and there is nothing in the 

 nature of the rock to ead us to expect any well marked system. 



By far the most remarkable water-bearing horizon is found at the 

 upper surface of the Cinninati shales, the dividing plane between 

 the Upper and Lower Silurian formations. The upper part of the 

 shale is little else than a stiff, compact, blue clay, and is the most im- 

 pervious stratum in this portion of the state. Above it there lies a 

 varying thickness of fissured, cavernous Niagara limestone, through 

 which the water descends till its progress is arrested by the clay from 

 the surface of which it flows out wherever opportunity offers. Along 

 the east side of the Green Bay and Rock river valley, the junction of 

 these strata is exposed at frequent intervals for one hundred and fifty 

 miles, so that hundreds if not thousands of springs, great and small, 

 mark this horizon. When a heavy mass of drift clay lies against the 

 line of junction, the water sometimes issues from layers of the lime- 

 stone, several feet higher, and where much loose rock has fallen from 

 the cliffs above, the stream runs in a concealed channel down the 

 slope, and appears to issue scores of feet, sometimes, below its true 

 point of exit from the strata. Yet; notwithstanding these facts, with 

 proper caution, the point at which the clay and limestone join, even 

 when concealed, may be traced approximately by this line of springs. 

 Many of these fountains are very large and strong; indeed, some 



