212 Canadian Record of Science. 



volcanic phenomena either of an explosive or quiet char- 

 acter, as may be determined by contact with water. The 

 underlying material may also be carried to the surface by 

 the agency of heated water, producing those quiet discharges 

 which Hunt has named crenitic. It is to be observed here 

 that explosive volcanic phenomena, and the formation of 

 cones, are, as Prestwich has well remarked, characteristic 

 of an old and thickened crust ; quiet ejection from fissures 

 and hydro-thermal action may have been more common in 

 earlier periods and with a thinner over-crust. 



(6) The contraction of the earth's interior by cooling 

 and by the emission of material from below the over-crust, 

 has caused this crust to press downward, and therefore 

 laterally, and so to effect great bends, folds, and plications ; 

 and these, modified subsequently by surface denudation, 

 constitute mountain chains and continental plateaus. As 

 Hall long ago pointed out, 1 such lines of folding have been 

 produced more especially where thick sediments had been 

 laid down on the sea bottom. Thus we have here another 

 apparent paradox, namely, that the elevations of the earth's 

 crust occur in the places where the greatest burden of de- 

 tritus has been laid down upon it, and where consequently 

 the crust has been softened and depressed. We must be- 

 ware, in this connection, of exaggerated notions of the 

 extent of contraction and of crumpling required to form 

 mountains. Bonney has well shown, in lectures delivered at 

 the London Institution, that an amount of contraction, al- 

 most inappreciable in comparison with the diameter of the 

 earth, would be sufficient ; and that as the greatest moun- 

 tain chains are less than <ruuth of the earth's radius in 

 height, they would, on an artificial globe a foot in diameter, 

 be no more important than the slight inequalities that 

 might result from the paper gores overlapping each other 

 at the edges. 



(7) The crushing and sliding of the over-crust implied in 

 these movements raise some serious questions of a physical 



1 Hall, (American Association Address, 1857, subsequently re- 

 published, with additions, as Contributions to the Geological History 

 of the American Continent,) Mallet, Eogers, Dana, Le Conte, &c. 



