PROCEEDINGS OF THE GEOLOGICAL SOCIET5T. 69 



cannot be assumed to have been constant ; and though it is known 

 that pure water will take up 2 grains per gallon of carbonate of 

 lime, the rain-water that acted on the limestone was not pure, but 

 had obtained from various sources a considerable but variable 

 quantity of carbonic acid, which would enable it to remove the lime- 

 stone far more rapidly. Moreover it was not at all clear that the 

 boulders had been left on a clean surface of limestone, but many of 

 them probably were left there when the Boulder- clay in which they 

 had been imbedded was washed away by streams of water not strong 

 enough to carry the boulders too. He thought that the breaking- 

 down of parts of the pedestal was due to the weight of the boulder 

 when denudation along joints and furrows had isolated and under- 

 mined small portions, and that if they had been dropped through 

 water the velocity would have been checked, or if, as was more 

 probable, left by a glacier, they would not have been dropped on to 

 the bare rock at all ; and he gave the example of boulders hurled by 

 a torrent against the wall of Gaping Gill, and only bruising the sur- 

 face of the rock, but not breaking it up. 



Mr. Tiddeman agreed in the main with Prof. Hughes as to the 

 Norber boulders. If the rate of waste of limestone could be ascer- 

 tained, that would only give us a minimum of the time elapsed. 

 But the hypothesis that the boulders had been dropped on pre- 

 existing pinnacles was highly improbable. The arrangement of the 

 boulders on a plateau of limestone with scratches beneath them, and 

 scratches on the parent rock to the north at a lower level, though 

 higher up the valley, these scratches being in the same straight line, 

 showed distinctly that the agent was land-ice. 



Dr. Hicks thought Mr. Mackintosh had failed to prove his theory. 

 Most of these areas must have been covered by Till. He showed by 

 a section how the boulders were imbedded in the Till. 



Prof. Bonnet said there were many theoretical difficulties in Mr. 

 Mackintosh's supposition, although it was remarkable how little 

 change had taken place in some regions since the disappearance of 

 the ice. History or tradition reached back for at least half the 

 period of 6000 years, and there was no hint of a refrigeration of 

 climate ; 10,500 years was required for the maximum of cold, if we 

 were to assign a lowering of temperature to precession ; while, if 

 we consult the tables of the values of the eccentricity of the earth's 

 orbit, we find the first high eccentricity was 100,000 years ago, 

 so that 6000 years from the present time the change due to this 

 would hardly be appreciable. 



3. " Notes on the Corals and Bryozoans of the Wenlock Shales 

 (Mr. Maw's Washings)." By Gr. R. Vine, Esq. Communicated by 

 Prof. P. Martin Duncan, M.B., F.E.S., Y.P.G.S. 



[Abstract.] 



The author briefly discussed the views of different writers upon 

 the systematic position of the genera Chastetes, Monticulipora, and 

 their allies, and also of the forms referred to the Polyzoa, and gave 



vol, XXXIX. (I 



