MOVEMENTS AXD DEFORMATIONS OF THE EARTH'S BODY. 539 



ward warping somewhere else in the 72% of the earth's surface covered 

 by the ocean, and so it is only the sum total of all changes in the sea- 

 bottoms and borders that effects the common level. Thus it happens 

 that, not^^ithstanding its instability and its completf^ subordination to 

 the lithosphere, the sea-level is the most convenient basis of reference, 

 and has become the accepted datum-plane. If there were some avail- 

 able mode of measuring the distance of points from the center of the 

 earth, it would give absolute data and absolute terms, and would reveal 

 much that is now micertain respecting the real movements of the sur- 

 face. For convenience, however, since absolute terms are impracti- 

 cable, the ordinary language of geology, which represents movements 

 as upward and do^^mward, according to their relations to the sea-level 

 or to the average surface, will be employed. Notwithstanding this 

 concession to convenience in the use of terms, it is of the greatest impor- 

 tance to form, and to constantly retain, true fundamental views. 



Fundamental conceptions. — The existence of any land at all is 

 dependent on the inequalities of the surface and of the density of the 

 lithosphere, for if it were perfectly spheroidal and equidense, the hydro- 

 sphere would cover it completely to a depth of about two miles. Not 

 only are inequalities necessary to the existence of land, but these inequali- 

 ties must be renewed from time to time, or the land area would soon, 

 geologically speaking, be covered by the sea. The renewal has been 

 made again and again in geological history by movements that have 

 increased the inequalities in the surface of the lithosphere. With each 

 such movement, apparently, the oceans have \\^thdra^^TL more completely 

 within the basins, and the continents have stood forth more broadly 

 and relatively higher, until again worn down. This renewal of inequali- 

 ties appears to have been, in its great features, a periodic movement , 

 recurring at long intervals. In the intervening times, the sea has crept 

 out over the lower parts of the continents, moving on steadily and 

 slowly toward their complete submersion, which would inevitably have 

 been attained if no interruption had checked and reversed the process. 

 These are the great movements of the earth, and in them lies, we believe, 

 the soul of geologic history and the basis for its grand divisions. The 

 reasons for this will appear as the history is followed, and its most poten- 

 tial agencies are seen unfolding themselves. At the same time, there 

 have been numerous minor surface movements in almost constant 

 progress. While these two classes of movements have been associated, 



