ADDBESS. 11 



named crenitic. It is to be observed here that explosive volcanic pheno- 

 mena, 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 comm.on 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,* 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 detritus 

 has been laid down upon it, and where consequently the crust has been 

 softened and depressed. We must beware, in this connection, of exagge- 

 rated 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, almost inappreciable in 

 comparison with the diameter of the earth, would be suJ9&cient ; and that 

 as the greatest mountain chains are less than y-Joth 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 move- 

 ments raise some serious questions of a physical character. One of these 

 relates to the rapidity or slowness of such movements, and the consequent 

 degree of intensity of the heat developed, as a possible cause of meta. 

 morphism of rocks. Another has reference to the possibility of changes 

 in the equilibrium of the earth itself as resulting from local collapse and 

 ridging. These questions in connection with the present dissociation of 

 the axis of rotation from the magnetic poles, and with changes of climate, 

 have attracted some attention,^ and probably deserve further considera- 

 tion on the part of physicists. In so far as geological evidence is con- 

 cerned, it would seem that the general association of crumpling with 

 metamoi'phism indicates a certain rapidity in the process of mountain- 

 making, and consequent development of heat, and the arrangement of the 

 older rocks around the Arctic basin forbids us from assuming any extensive 

 movement of the axis of rotation, though it does not exclude changes to 

 a limited extent. I hope that Professor Darwin will discuss these points 

 in his address to the Physical Section. 



' Hall (American Association Address, 1857, subsequently republished, with 

 additions, as Contribttticms to the Geological History of the AvieHcan Contine7it)r 

 Mallet, Kogers, Dana, Le Conte, &c. 



' See recent papers of Oldham and Fisher in Geological Magazine and PMlo- 

 sojfhical MagaHne, July 1886. Also Leroche, Mevol. Polaires. Paris, 1886. 



