42 ADDEESS TO THE BRITISH ASSOCIATION, 1881. 



argued by our new Secretary, Mr. Bonney irregular 

 movements of upheaval along lines athwart the valleys. 



Passing from lakes to mountains, two rival theories 

 with reference to the structure and origin of volcanoes 

 long struggled for supremacy. 



The more general view was that the sheets of lava and 

 scorise which form volcanic cones such, for instance, 

 as JEtna or Vesuvius were originally nearly horizontal, 

 and that subsequently a force operating from below, and 

 exerting a pressure both upwards and outwards from a 

 central axis towards all points of the compass, uplifted 

 the whole stratified mass and made it assume a conical 

 form, giving rise at the same time, in many cases, to a 

 wide and deep circular opening at the top of the cone, 

 called by the advocates of this hypothesis a ' crater of 

 elevation.' 



This theory, though, as it seems to us now, it had 

 already received its death-blow from the admirable 

 memoirs of Scrope, was yet that most generally adopted 

 fifty years ago, because it was considered that compact 

 and crystalline lavas could not have consolidated on a 

 slope exceeding For 2. In 1858, however, Sir C. Lyell 

 conclusively showed that in fact such lavas could con- 

 solidate at a considerable angle, even in some cases at 

 more than 30, and it is now generally admitted that 

 though the beds of lava, &c., may have sustained a 

 slight angular elevation since their deposition, still in 

 the main, volcanic cones have acquired their form by 

 the accumulation of lava and ashes ejected from one or 

 more craters. 



The problems presented by glaciers are of very great 

 interest, n 1843 Agassiz and Forbes proved that the 



