ADDRESS. 23 



new secretary, Professor Bonney — irregular movements of nplieaval along 

 lines athwart tlie 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 ^tna 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, ujDlifted 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 com- 

 pact and crystalline lavas could not have consolidated on a slope 

 exceeding 1° or 2°. In 1858, however. Sir C. Lyell conclusively showed 

 that in fact such lavas could consolidate 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. In 

 1843 Agassiz and Forbes proved that the centre of a glacier, like 

 that of a river, moves more rapidly than its sides. But how and why do 

 glaciers move at all ? Rendu, afterwards Bishop of Anne§y, in 1841 

 endeavoured to explain the facts by supposing that glacier ice enjoys a 

 kind of ductility. The 'viscous theory' of glaciers was also adopted, 

 and most ably advocated, by Forbes, who compared the condition of a 

 glacier to that of the contents of a tar barrel poured into a sloping 

 channel. We have all, however, seen long narrow fissures, a mere fraction 

 of an inch in width, stretching far across glaciers — a condition incom- 

 patible with the ordinary idea of viscosity. The phenomenon of regelation 

 was afterwards applied to the explanation of glacier-motion. An obser- 

 vation of Faraday's supplied the clue. He noticed in 1850 that when 

 two pieces of thawing ice are placed together they unite by freezing at 

 the place of contact. Following up this suggestion Tyndall found that 

 if he compressed a block of ice in a mould it could be made to assume 

 any shape he pleased. A straight prism, for instance, placed in a groove 

 and submitted to hydraulic pressure, was bent into a transparent semi- 

 circle of ice. These experiments seem to have proved that a glacial 

 valley is a mould through which the ice is forced, and to which it will 

 accommodate itself, while, as Tyndall and Huxley also pointed out, the 

 ' veined structure of ice ' is produced by pressure, in the same manner as 

 the cleavage of slate rocks. 



