34 WALKS AND TALKS. 



In regions of deep Drift and abundant water-basins, the 

 supplies of spring-water are sometimes sufficient to meet the 

 demands of towns and cities. The city of Ann Arbor, with 

 its ten thousand of population, is thus supplied with nearly 

 five hundred thousand gallons daily. This is obtained from 

 two groups of springs, and distributed through the city in the 

 usual way. Five times this amount could be had, if needed. 



Now let us consider springs in another light. We have al- 

 ready reflected that the percolating water takes some substances 

 in solution from the surface. It must take up much more in 

 leaching through the sands. This is the reason why most 

 sands are composed chiefly of insoluble constituents. Their 

 soluble constituents have been leached out. But there re- 

 main still, in many regions, some soluble limestone pebbles or 

 larger masses which have not yet been dissolved, and the 

 water is continually diminishing the amount of these. * Now, 

 first of all, consider that this little fact is a demonstration 

 that the present order of things has not existed from eternity. 

 If it had, all the soluble substances would have disappeared 

 long ago in fact, an eternity since. The time, therefore, 

 since this Drift was brought here is only a finite number of 

 years. That is a positive datum. 



Water that has dissolved much limestone is "hard." Hence, 

 many spring and river waters are hard. The water of a pond 

 may be softer, because a large proportion of it has been di- 

 rectly rained in, or supplied by surface drainage from the sur- 

 rounding land. Of course, the hardness of underground water 

 depends upon the amount of limestone pebbles and grains 

 with which it has come in contact. Aside from any supply 

 of limestone from neighboring ledges, the amount of limestone 

 in the Drift depends on the amount transported from the north- 

 ern regions which supplied the material of the Drift at each 

 particular place. Some northern regions supplied much lime- 

 stone, and others little. Hence, in Michigan, Ohio, and In- 

 diana, well and spring waters are hard, while in New England 

 and Pennsylvania they are comparatively soft. 



Under-ground water often experiences great pressure. In 



