ANNIVERSAEY ADDEESS OF THE PRESIDENT. 



6i 



II. The absence, with a few not very important exceptions, in 

 ocean tracts, of islands formed of stratified rocks such as compose 

 the bulk of the continents, and the fact that nearly all oceanic 

 islands are volcanic, and consequently sach as may have been built 

 up from oceanic depths by the accumulation of volcanic discharges. 



III. The absence of deep-sea deposits in the rocks of conti- 

 nental areas. 



lY. The agreement between the distribution of plant- and animal- 

 iife and the present arrangement of land-areas. 



I propose to consider each of these arguments in order, though 

 the last is that to which it will be necessary to give the most 

 attention. 



I. Greater Density of Infra-oceanic Crusts. — The first argument is 

 one to which some importance is attached by Prof. Dana *. But it 

 is only founded on a few observations in India and the neighbouring 

 islands, and was suggested by its author. Archdeacon Pratt, as a 

 probable hypothesis to account for some anomalies in the results of 

 pendulum observations. The principal force of the contention 

 ;appears to lie in the circumstance that if the crust of the earth now 

 below the ocean had been exposed to denudation, and consequently 

 transport, its exceptional density could not have been preserved, 

 because the action of rivers and currents would in the course of ages 

 have mingled its detritus with that of the present continental areas. 

 It has also been urged that, owing to the contact of cold oceanic 

 waters, the crust beneath the ocean has become thicker and more 

 rigid. This view, however, depends on the hypothesis of a fluid 

 layer beneath the solid crust ; and although there is much to be said 

 in favour of such a view, it is impossible to accept it as a proved 

 theory and to use it as a basis for argument. Indeed the next 

 point to be noticed, the presence of volcanic islands in many parts 

 of the ocean, is really antagonistic to the idea of a uniformly thicker 

 ixnd more rigid crust beneath the ocean-bed, because the occurrence 

 of volcanoes indicates areas and lines of weakness. If the infra- 

 oceanic crust were much more rigid and thicker than the infra-con- 

 tinental, volcanoes should be confined to the area occupied by the 

 latter ; but this is not the case. That volcanoes should be most 

 numerous near the boundaries between continental and oceanic 

 .areas is natural. 



II. The Volcanic Origin of Oceanic Islands. — This, the second 



* Manual of Geology, 3rd ed. p. 815. 



