PRELIMINARY OUTLINE. 9 



solution of the soluble part usually loosens the insoluble, and renders 

 it an easy prey of the surface waters. These transport the loosened 

 material to the valleys and at length to the great basins, meanwhile 

 rolling and grinding it and thus reducing it to rounder forms and a 

 finer state, until at length it reaches the still waters or the low gradients 

 of the basins and comes to rest. The hydrosphere is therefore both 

 destructive and constructive in its action. As the beds of sediment 

 which it lays down follow one another in orderly succession, each later 

 one lying above each earlier one, they form a time record. And as 

 relics of the life of each age become more or less imbedded in these 

 sediments, they furnish the means of following the history of life from 

 age to age. The historical record of geology is therefore very largely 

 dependent upon the fact that the waters have thus buried in systematic 

 order the successive life of the ages. Aside from this, the means of 

 determining the order of events of the earth's history are limited and 

 more or less uncertain. 



The special processes of the hydrosphere in its various phases will 

 be the subject of discussion hereafter (Chaps. Ill, IV, VI). Suffice it here 

 to recognize its great function in the constant degradation of the land> 

 and in the deposition of the derived material in orderly succession in 

 the basins. 



Chief horizons of activity. — The great horizons of geological activity 

 are (1) the contact zone between the atmosphere and the hydrosphere, 

 chiefly the surface of the ocean, (2) the contact zone between the hydro- 

 sphere and the lithosphere, chiefly the shore belts, and (8) the contact 

 zone of the atmosphere and surface waters, with the face of the conti- 

 nents. It is in these three zones that the greatest external work is 

 being done and has been done in all the known ages. 



///. The Lithosphere. 



The atmosphere and hydrosphere are rather envelopes or shells 

 than true spheres, though in some degree both penetrate the lithosphere. 

 The lithosphere, on the other hand, is a nearly perfect oblate spheroid 

 with a polar diameter of 7899.7 miles, and an equatorial diameter of 

 about 26.8 miles more. Its equatorial circumference is 24,902 miles, 

 its meridional circumference 24,860 miles, its surface area 196,940,700 

 square miles, its volume 260,000,000,000 cubic miles, and its average 

 specific gravity about 5.57. The oblateness of the spheroid is an ac- 

 commodation to the rotation of the earth, the centrifugal force at the 



