418 A. Blytt on the probable Can^e of 



tion, such as it would if it were allowed to act for a longer 

 time. This applies not only to tension, but generally ; and 

 hence it comes about that wires slacken in course of time, and 

 that beams bend little by little. A thread is worn out by less 

 force when the pressure is long continued than when it is 

 applied for a shorter time.'^ 



It seems to me that here we have a force which may be ~^ 

 capable of effecting displacements in the solid earth. I believe \ 

 that this is " the unknown force from below " which has ele- i 

 vated the mountains of Western North America, and to which ^ 

 Dutton 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. But 

 the solid earth offers resistance to chano-e 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 opposite direction. The solid earth pro- 

 bably 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 perhaps take place more spasmodically, 

 with intervening periods of rest, during which new tension is 

 S3t up. 



" The elevation of mountains/' says A. Geikie (' Text-book 

 of Geology,' 1882, p. 917), " is in most cases due to a long 

 succession of such movements;" and (/. c. p. i)ll)) " the ele- 

 vation of mountains, like that of continents, has been occa- 

 sional, and, so to speak, ])aroxysmai." Upheavals of the 

 crust take place repeatedly along the same fissure (see, e. g., 

 Brogger, Bildungsgescluchte des Kristianiafjords, 1886, p. 78). 

 Something of the same kind occurs in volcanic eruptions. 

 Volcanoes rest for a shorter or longer time between the dif- 

 ferent eruptions. Basaltic layers alternate with sedimentary 

 deposits. Earthquakes are a consequence of a tension set up, 

 to which the crust suddenly yields. All this indicates that 

 the crust of the earth does not immediately accommodate 

 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 place, in 

 both the Old and the New World, in which melted masses 

 burst forth from numerous fissures and overflowed thousands 



