Geological Society. 73 



(fig. 2) represent in section two sides of a cube of a viscous solid, and 

 if, by "shearing" parallel to these planes, C D be brought to the 

 position C D', relatively to A B supposed to remain at rest, and if this 

 process be continued until the material breaks, it breaks parallel to 

 ABandC'D'. 



The appearances presented by the specimens in Mr. Kirkaldy's 

 museum attracted my attention by their bearing on an old contro- 

 versy regarding Forbes's theory of glaciers. Forbes had main- 

 tained that the continued shearing motion which his observations 

 had proved in glaciers, must tend to tear them by fissures parallel 

 to the surfaces of " shearing." The correctness of this view for a 

 viscous solid mass, such as snow becoming kneaded into a glacier, 

 or the substance of a formed glacier as it works its way down a 

 valley, or a mass of debris of glacier-ice, reforming as a glacier after 

 disintegration by an obstacle, seems strongly confirmed by the ex- 

 periments on the softer metals described above. Hopkins had argued 

 against this view, that, according to the theory of elastic solids, as 

 stated above, and represented by the first diagram, the fracture 

 ought to be at an angle of 45° to the surfaces of " shearing." There 

 can be no doubt of the truth of Hopkins's principle for an isotropic 

 elastic solid, so brittle as to break by shearing before it has become 

 distorted through more than a very small angle ; and it is illus- 

 trated in the experiments on brittle sealing-wax and hardened steel 

 which I have described. The various specimens of fractured elastic 

 solids now exhibited to the Society may be looked upon with some 

 interest, if only as illustrating the correctness of each of the two 

 seemingly discrepant propositions of those two distinguished men. 



GEOLOGICAL SOCIETY. 

 [Continued from vol. xxxvii. p. 311.] 



Nov. 25th, 1868.— Prof. T. H. Huxley, LL.D., F.R.S., 

 President, in the Chair. 



The following communications were read : — 



1. "On Floods in the Island of Bequia." By G. M. Browne, 

 Esq. Communicated by the Secretary of State for Foreign Affairs. 



On the 17th of March, at 8 o'clock p.m., a steady strong wave was 

 seen bearing down upon Admiralty Bay; it had no perceptible 

 crest, and was three feet in height ; it encroached upon the land to 

 distances varying from 70 to 350 feet. A second, smaller wave 

 followed. No shock of an earthquake was felt. 



2. " Description of Nga Tutura, an Extinct Yolcano in New 

 Zealand." By Capt. F. W. Hutton, F.G.S. 



This volcano is situated on the west coast of the North Island of 

 New Zealand, between Raglan and the mouth of the River "Waikato. 



A section of 15 miles is exposed along the coast,* which trends 

 in a north-west and south-east direction, showing beds of Me- 

 sozoic age forming a synclinal trough between the south head of 

 Waikato and Otehe Point, and descending below the sea-level at Wai- 

 kawau. Upon them lie Tertiary strata, following the same synclinal 



