50-(20) MINERAL PHYSIOLOGY. 



the whole Atlantic belt to the northward, and how in 

 the valley of the Mississippi, where the present outcrops 

 of these ancient rocks are eroded, as in the east; we find 

 beneath sandstones of later times portions of the old de- 

 cayed mantle which had escaped erosion until thus pro* 

 tected. Such a process of decay has always been going 

 on at the earth's surface, and is even now slowly but 

 certainly wasting the hardest rocks. 



The usual eroding agents, ice and water, are inade- 

 quate to remove great areas of hard rocks unless these 

 have previously been softened by decay, and a careful 

 study of the question led me long since to maintain that 

 the chemical decomposition of rocks is a necessary pre. 

 iiminary to glacial and erosive action, which has removed 

 only already softened materials. From this it follows 

 that the forms and outlines of the sculptured surface 

 thereby exposed would be determined by the varying 

 depths to which the process of shbaerial decay had already 

 penetrated the once firm and solid rock. The boulders, 

 the rounded bosses and hillocks, and the closed basins, 

 which are so conspicuous in glaciated regions of crystal- 

 line rocks, are due not primarily to glacial erosion, as 

 some have taught, but to previous decay, softening 

 portions of the rocks, the subsequent removal of which 

 has given rise to these outlines. Some of these contours 

 now observed were thus sculptured in very remote geo- 

 logical ages, and later glacial action lias done little more 

 than to groove and polish them. 



We have already noticed the fact that in this decom- 

 position a large proportion, equal in many cases to more 

 than one half of the weight of the rock, passes into solu- 

 tion in the superficial waters. Of these dissolved mat- 

 ters a certain portion, filtering downward, enters into the 

 terrestrial circulation, but by far the large]- part find their 

 way directly into what we have designated the oceanic 

 circulation, which includes alike the rainfall, rivers, 

 lakes, and the sea itself. The dissolved silica, lime and 

 magnesia, have contributed greatly to the building up of 

 stratified rocks, including besides limestones and dolo- 

 mites many silicated and silicious deposits ; while by the 

 action of the dissolved magnesian and alkaline salts the 

 composition of the ocean's waters has in successive ages 

 been greatly modified. 



