40 GEOLOGICAL SURVEY OF WISCONSIN. 



leaves, shells, sticks, etc., are often incrusted in such manner as to 

 preserve beautifully their peculiar markings. 



In many cases the oxide of iron is thus thrown down, discoloring 

 everything over which the water runs. The quantity of iron thus 

 deposited does not depend so much upon the absolute quantity of 

 iron in the composition of the water as upon the feebleness with 

 which it is held in solution. Such springs are called chalybeate, or 

 iron springs. 



Sulphuretted hydrogen, which gives the sulphur taste and odor to 

 many springs, is supposed to result from the decomposition of or- 

 ganic (animal or vegetable) matter mostly of vegetable origin. Such 

 springs are called sulphur springs, and we have several examples in 

 the state. 



The rock strata are clearly shown, by the evidence of marine fossil 

 shells and corals, to have been originally deposited in the waters of 

 an ancient sea, from which they have been elevated to form our 

 present continent; and it is not, therefore, wonderful that they should 

 yield to the percolating waters those elements which we find in 

 mineral waters. The supply might be supposed to diminish as ages 

 pass by, but never to become exhausted. 



All natural waters may be regarded as "mineral waters" for 

 none are absolutely pure. Indeed it is probable that pure water, such 

 as can only be obtained by careful distillation, would not sustain 

 either animal or vegetable life. Even rain water is found to contain 

 minute quantities of salt, of ammonia, soda, lime, and organic matter, 

 with traces of many other substances. It is mainly from the mineral 

 matters dissolved in water that plants derive their solid materials con- 

 stituting the ash. The silica, though small in amount in water, is of 

 great importance to the growth of plants, especially the grasses, in- 

 cluding the cereals, etc. ; all our waters contain silica. 



Springs, whose sources are near the surface, are apt to contain mat- 

 ters resulting from the decay of organic matter, and other deleterious 

 material. Those which come from greater depths, and .have passed 

 over greater distance, are supposed to be of greatest value ; the organic 

 matter having been absorbed in passing through the soil and rocks. 

 Such springs usually assume a temperature showing little change 

 from winter to summer, and which correspond, with the mean annual 

 temperature of the place. 



Several of these waters, notably those of Fond du Lac, Sparta, and 

 Watertown, are known as magnetic; the iron tubes through which 

 they flow, possessing the magnetic property in a high degree. Whether 

 this magnetic quality has any medicinal value remains yet to be de- 



