ANNUAL REPORT FOR 1873. 39 



of water absorbed by the roots of plants, with its mineral contents, 

 and then evaporated from the leaves, is enormous. 



Rain water soon penetrates the soil and finds its way through 

 crevices and pores of the more solid rock strata, down to very con- 

 siderable depths, dissolving such substances as are soluble and carry- 

 ing them to distant places. It is here we must look for the origin 

 of all mineral waters, and for the cause of a large share of the de- 

 nudation which in the lapse of many ages has become so very con- 

 siderable. 



Knowing this annual quantity of water-supply and outflow, with 

 the amount of mineral matter it carries, we lack only a knowledge of 

 the amount of matter mechanically suspended in running water, de- 

 rived from the loose soil and from abraded rocks, to be able to cal- 

 culate the time required for the excavation of any given valley whose 

 dimensions are known. 



The gradual decay and wearing away of rocks is not confined to the 

 limestones and softer rocks, but includes many of the older and harder 

 kinds, especially such as contain feldspar and similar minerals. Rocks 

 are always penetrated with water and they hold in their pores an 

 average of five or six per cent, by weight ; a hundred pounds being 

 capable of absorbing five or six pounds of water. As the clouds 

 afford a constant supply which is continually removed in under- 

 ground currents, springs and rivers, carrying with it the mineral 

 matter taken up, we have a perpetually recurring cause of geological 

 change. During the progress of water through the different rocks, 

 meeting with different chemical elements, many chemical changes 

 probably occur in the substances taken up, and it is only the final 

 result of all these changes that we find in the mineral spring as it 

 appears at the surface. Any considerable change of the course of the 

 water among the rocks would, therefore, lead to changes in the com- 

 position or combination of ingredients it contains. 



"Water when pure has but little power to dissolve mineral matters, 

 but when combined with carbonic acid, this dissolving power is very 

 much increased. When waters holding these substances reach the 

 open air they give off the excess of carbonic acid, and hence, by losing 

 a portion of their dissolving power, become unable to hold them, and 

 they are deposited forming a coating to sticks and stones over which 

 they may run. 



If this deposit consists chiefly of lime, and accumulates with con- 

 siderable rapidity, such springs are known as petrifying springs, or 

 lime springs, of which we have many examples. The deposit often 

 forms considerable conical mounds surrounding the spring; and moss, 



