x EARTH CHANGES & EVOLUTION 183 



of weakness, always near existing mountains ; and further, 

 if this cause of elevation be supplemented by the con- 

 tinual subsidence of large areas along the margins of all the 

 continents by the weight of new deposits producing a pres- 

 sure on the liquid interior, which must result in upward 

 pressures elsewhere, then it seems possible that a combina- 

 tion of these causes may be sufficient. 



Yet another cause of elevation has recently been demon- 

 strated. After many unsuccessful attempts, the actual 

 existence of semi - diurnal lunar tides within the earth's 

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

 generate a vast amount of heat, culminating at the bi-monthly 

 periods of maximum effect. The heat thus produced would 

 be greatest where the under surface of the crust was most 

 irregular, that is, under the land surfaces, and especially 

 under the " roots of mountains " projecting below the general 

 level. Their cumulative results would, therefore, add to the 

 upward forces produced by contraction along lines of weak- 

 ness. 1 



But whether the various forces here suggested have 

 been the only forces in operation or not, the fact of the 

 repeated slow elevations and depressions of the earth's sur- 

 face is undoubted. The most general phenomenon seems 

 to have been the very slow elevation of great beds of strata, 

 deposited one above another along the coasts of a con- 

 tinental mass, or sometimes along the shores of inland seas ; 

 immediately followed by a process of denudation of this 

 surface by rain and rivers, which, as the elevation con- 

 tinued, carved it out into a complex series of valleys and 

 ridges till it ceased to rise farther. The denudation con- 

 tinuing, the whole mass became worn away into lowland 

 plains and valleys. Then, after a long period of quiescence, 

 subsidence began, and as the land sank beneath the water 

 new deposits were laid down over it. Sometimes repeated 

 elevations and depressions of small extent occurred ; while 

 at very long intervals there was great and long-continued 

 subsidence, and, while deeply buried under newer strata, the 



1 This sketch of the internal structure of the earth, as affecting elevation and 

 depression of its surface, is fully discussed in Mr. O. Fisher's Physics of the 

 Earth's Crust, a popular abstract of which is given in my Studies Scientific and 

 Social, vol. i. chap. iii. 



