346 DYNAMICAL GEOLOGY. 



2. Changes in the position of the earth's axis. — If a change should take 

 place in the position of the earth's axis, through changes of level in the earth's 

 crust, the coast-lines of the earth would be throughout affected by the new- 

 adjustment of the water level. Physicists have very nearly relieved geology 

 of this source of doubt, by the decision that an effective change of this kind 

 is exceedingly improbable (page 255). 



3. A change in the level of the land. — By the law of gravitation, elevated 

 lands attract, and thus draw the mobile waters of the ocean toward them 

 to a height dependent on their mass and distance. Consequently the sea- 

 margin of all coasts is more or less displaced, and much so, wherever the 

 land mass adjoining rises high above it. It has been calculated that from 

 this cause the sea level at the center of the Eurasian continental mass is 

 about 2900 feet above the sea at its margin (R. S. Woodward) ; at the center 

 of the Australian mass, about 400 feet (G. G. Stokes, 1849, 1887) ; of the 

 great plateau of India, 1000 feet, but under the volcanic mountain of Maui, 

 Haleakala, 10° in mean slope, only 10 feet. 



The facts make it evident that the water-line along nearly all coasts, and 

 especially on the west coast of North and South America, must be very 

 largely moved inland by the mountain chains ; and that, through geological 

 time, changing levels have always been changing the water-lines. It is to be 

 observed, furthermore, that this inland drawing of the ocean's water dimin- 

 ishes the height of the mountains above the sea. The error is on the side of 

 too little height. 



The piling of ice over the land in a glacier epoch has a like effect, but 

 with material having about two fifths the gravity of the ordinary land 

 material. Were the ice of a glacial epoch to be accumulated about the 

 poles, and thus make a polar ice-cap or meniscus thousands of feet high, 

 the ocean level would be changed through all latitudes to the equator. This 

 cause has been thought sufficient to explain apparent subsidences in the 

 hemisphere so capped. 



But since the change of water level from the equator to the pole would 

 follow a geometric ratio, admitting of mathematical calculation, the accord- 

 ance of the theory with actual facts is easily tested. In eastern America 

 the subsidences closing the Glacial period supposed to be thus accounted 

 for by Croll have no correspondence with the required heights. Moreover, 

 observations have proved that there was no such polar ice-cap in the Glacial 

 period. 



4. Abstraction of ivater from the ocean. — Further, the making of great 

 continental accumulations of ice would lower the level of the ocean and tend 

 to raise the apparent level of the land. 



With the above cautionary considerations in view, noting that the obser- 

 vations about ice relate only to glaciated regions, that the error from the 

 attraction of the land is on the side of too little height, and that sea-bottom 

 changes of level affect all coast-lines alike, the following facts may be ac- 

 cepted as proof of changing levels over the earth's surface. 



