286 TIME REQUIRED FOR CHANGES OF LEVEL, chap. xir. 



last 400 years.* But, granting that in these and some ex- 

 ceptional cases (none of them as yet very well estahlished) 

 the rising or sinking has, for a time, been accelerated, I do 

 not believe the average rate of motion to exceed that above 

 proposed. Mr. Darwin, I find, considers that such a mean 

 rate of upheaval would be as high as we could assume for 

 the west coast of South America, where we have more evidence 

 of sudden changes of level than anyAvhere else. He has 

 not, however, attempted to estimate the probable rate of 

 secular elevation in that or any other region. 



Little progress has yet been made in divining the most 

 probable causes of these great movements of the earth's crust ; 

 yet what little we know of the state of the interior leads us 

 to expect that the gradual expansion or contraction of large 

 portions of the solid crust may be the result of fluctuations in 

 temperature, with which the existence of hundreds of active 

 and thousands of extinct volcanoes is probably connected. 



It is ascertained that solid rocks, such as granite and 

 sandstone, expand and contract annually, even under such 

 a moderate range of temperature as that of a Canadian 

 winter and summer. If the heat should go on increasing 

 through a thickness, say only of ten miles of the earth's 

 crust, the gradual upheaval of the incumbent mass may 

 amount to many hundreds of feet; and the elevation may 

 be carried still farther, b}' the complete fusion of part of the 

 inferior rocks. 



According to the experiments of Dcville, the contraction 

 of granite, in passing from a melted, or, as some would say, its 

 plastic, condition, to a solid state, must be more than ten 

 per cent.f So that we have at our command a source of 

 depression on a grand scale, at every period Avhen granitic 



■-■■■ Seasons with the Sea-IIorscs, p. 202. 



•j- Bulletin de la Societe Geologique, 2d scries, vol. iv. p. 1312. 



