336 ON THE MOVEMENTS OF THE EARTHS CRUST. 



effecting (Usplacemeuts in the solid earth. I believe that this is " the 

 unknown force from below" which has elevated the mountains of west- 

 ern North America, and to which Button appeals. The sidereal day 

 increases very slowly. The sea adjusts itself in accordance with the 

 smallest change in the length of the day and rises slowly under high 

 latitudes. Bnt the solid earth offers resistance to change of form, and 

 begins to give way only when the tension reaches a certain amount. 

 When this period has arrived the crust also begins to rise under high 

 latitudes. Under lower latitudes the movement takes place in the oppo- 

 site direction. The solid earth probably is a little behind the sea in its 

 movements, and while the sea moves evenly and uninterruptedly, the 

 change of form in the solid earth must perhai)S take place more spas- 

 modically, with intervening periods of rest, during which new tension 

 is set up. 



"Tlieelevation of jnountains," says A.Geikie (' Text-book of Geology,' 

 1882, p. 917), is in most cases due to a long succession of such move- 

 ments ;" and {I. c. p. 919) " the elevation of mountains, like that of con- 

 tinents, has been occasional, and so to speak, paroxysmal." Upheavals 

 of the crust take place repeatedly along the same fissure (see, e. </., 

 Brogger, Bildimgsgeschichte des Eristianiafjords, 1886, p. 78). Some- 

 thing of the same kind occurs in volcanic eruptions. Volcanoes rest 

 for a shorter or longer time between the different eruptions. Basaltic 

 layers alternate with sedimentary deposits. Earthquakes are a conse- 

 quence of a tension set up, to which the crust suddenly yields. All 

 this indicates that the crust of the earth does not immediately accom- 

 modate itself to the forces, but that it yields only when the constantly 

 increasing pressure has approximated to a certain amount. It seems 

 moreover to follow from geological investigations that there are periods 

 in the earth's history when changes have taken place on a larger scale 

 than usual. In his "Text-book" above cited (pp. 197,198) A. Geikie 

 refers to the great eruptions ("fissure-eruptions") which have taken 

 ])lace in both the Old and the New World, in which melted masses 

 burst forth from numerous fissures and overflowed thousands of square 

 miles. The Vulcauism of the jiresent day seems feeble in comparison 

 with these gigantic eruptions. 



We will now pass to the inquiry whether these changes in the form 

 of the earth may stand in any relation of dependency to the periodical 

 variations of the eccentricity of the earth's orbit. We start from 

 the fact that Thomson and Tait are right when they say that the tidal 

 wave is the most powerful of the forces which contribute to change the 

 length of the day. But besides the tidal wave of the sea, the interior 

 friction accepted by Darwin, (" the bodily tides ") is also effective. Both, 

 of course, are dependent upon the distance of the sun and moon ; and 

 we therefore examine whether the tidal action of these bodies upon the 

 earth varies with the eccentricity of the earth's orbit. It appears from 



