EARTH CHANGES A:N'D EVOLUTION 197 



covers them preserves uniform temperature, whicli, at the bot- 

 tom, is not much above the freezing point of sea-water. We may 

 conclude, therefore, that they are very nearly stable. Pen- 

 dulum experiments show that the crust below these oceans is 

 more dense than the subaerial crust, due, probably, to the uni- 

 form pressure and temperature they have been subject to for 

 geologic periods. We may assume, therefore, that they do not 

 become crumpled or distorted by the contraction of the liquid 

 earth beneath them. The great plains of Eussia, mostly of 

 Triassic and Jurassic age, consist of nearly horizontal strata, 

 while the Alps of Central Europe are greatly upheaved and 

 contorted ; and the same difference between adjacent areas is 

 found in the United States, and most probably in all the great 

 continents. 



Mathematical physicists have calculated the possible up- 

 heavals that could be produced by a shrinking crust at prob- 

 able rates of contraction, and have declared them to be too 

 small to account for the elevation of the existing land- 

 masses above the ocean floors, that is, for the whole differences 

 of height of the land surfaces. But if, as the Rev. O. Eisher 

 suggests, the oceanic basins were formed at an early stage of 

 the earth's consolidation, by the separation from it of the moon 

 in the way described by Sir George Darwun and accepted by 

 Sir Robert Ball ; and if the whole wrinkling effect of contrac- 

 tion is concentrated on a few lines or areas of weakness, al- 

 ways near existing mountains ; and further, if this cause of 

 elevation be supplemented by the continual subsidence of large 

 areas along the margins of all the continents by the weight of 

 new deposits producing a pressure on the liquid interior, which 

 must result in upward pressures elsewhere, then it seems pos- 

 sible that a combination of these causes may be sufficient. 



Yet another cause of elevation has recently been demon- 

 strated. After many unsuccessful attempts, the actual ex- 

 istence of semi-diurnal lunar tides ivithin iJic earth's interior 

 has been proved; and such tides must, it is said, generate a 

 vast amount of heat, culminating at tlio bi-monthly periods of 



