Geological Society of London. 235 



temperature of a particular layer, causes it to expand, and so to put 

 a strain upon the layer above, and then to rupture it. The broken 

 part spreads out, reunites by regelation, and then receiving the heat 

 from the layer below again expands and ruptures the layer next 

 above. Thus the movement is from the base upwards, rather than 

 from the surface downwards. 



The author estimates that the ice-sheet in Norfolk was only about 

 400 feet thick, because Boulder-clay does not appear above that 

 level, but onlj' coarse Boulder-claj^ ; in North Yorkshire it extends 

 up to about 900 feet. The author considers that the shell-beds of 

 Moel Tryfaen were not deposited under water, but thrust up-hill by 

 this advancing ice-sheet. 



3. " Soil-cap Motion." By E. W. Coppinger, Esq. Communicated 

 by the President. 



The author described numerous cases in Patagonia where the 

 stumps, etc., of trees are to be seen in the mai'ginal waters of" the 

 sea and of lakes. These, together with stones and rocks, sometimes 

 simulating perched blocks, he considers to have been brought down 

 by the motion of the soil-cap — a thick spongy mass resting upon 

 rock often worn smooth by the action of ice, and so sliding down 

 the more easily under the influence of vegetation. The appearances 

 are not unlike those due to subsidence ; but he points out that all 

 the evidence is in favour of recent upheaval, instead of subsidence. 



II.— April 6, 1881.— J. W. Hulke, Esq., F.E.S., Vice-President, 

 in the Chair. — The following communications were read : — 



1. " The Microscopic Chai'acters of the Vitreous Rocks of Montana, 

 U.S." By F. Rutley, Esq., F.G.S. With an Appendix by James 

 Eccles, Esq., F.G.S. 



The specimens described were collected by Mr. J. Eccles, F.G.S. 

 They consist of various obsidians and rhyolites, some of them 

 porphyritic or spherulitic, which appear to throw some light on the 

 epoch at which these structures have been set up. In a black 

 porphyritic obsidian is a crystal which the author believed to be 

 olivine. Zirkel has already noticed the occurrence of this mineral 

 in a trachyte. The structure of some of the above indicates that it 

 is extremely difficult to draw hard and fast lines between trachytic 

 rhyolites and felstones. A tuff containing fragments of a rhyolite, 

 some perlitic, was also described. The spaces included within the 

 boundary of some of these perlitic cracks exhibit depolarization and 

 sometimes interference-crosses. The author considered these to be 

 the result of strain in contraction, and connected with incipient 

 crystallization. 



Andesites, from two localities in the northern part of the Yellow- 

 stone district, were also described. 



In an appendix Mr. Eccles briefly described the geology of the 

 region from which the above specimens wei'e collected, referring for 

 greater detail to the memoirs of Dr. Hayden and his fellow-workers. 

 In the Yellowstone-Park region trachyte and obsidian (the latter 



