■206 Geological Society : — 



finds an expression for the period, o£ a different type from 

 mine, and hence concludes that my analysis is not suitable 

 for two horizontal dimensions. He forgets, though, that 

 while my waves are of type 



. qiT.r . ririf 

 u = sm - — sin —~ , 

 a b 



it being the velocity in the direction of a, and the sides being 

 given by y = and y = b, his are of the type 



. CllTX TTTll 



u= sm COS —r^ . 



a b 



I do not see why our results should agree as our boundary 

 equations differ. I also do not think much weight can be 

 placed on conclusions based on the assumption that there is 

 slipping at the sides, an assumption which manifestly does 

 not correspond with facts. 



On examining my results it certainly struck me as re- 

 markable that the influence of the sides should be so small. 

 But this seems borne out by facts, although the experiment 

 is not decisive. On reducing the breadth to one-half its 

 former value (cf. page 102 of my former paper) the loga- 

 rithmic decrement increased only 10 per cent. Had the 

 discrepancy been mainly due to the sides, I should have 

 expected a larger change. 



XXI. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from vol. xviii. p. 937.] 



April 28th, 1909.— Prof. W. J. Sollas, LL.D., Sc.D., F.R.S., 

 President, and afterwards Prof. W. W. Watts, Sc.D., F.lt.S., 

 Vice-President, in the Chair. 



fTVHE following communications were read : — 



1. ' The Boulders of the Cambridge Drift.' By Robert Heron 

 Rastall, M.A., F.G.S., and James Eomanes, B.A. 



For several years past a large number of boulders have been 

 collected from the Glacial drifts of Cambridgeshire, and from the 

 post- Glacial gravels which have been derived from the drifts. 

 These specimens have been classified geographically and then 

 subjected to a careful petrological examination, with a view to the 

 determination of their origin. Some special collections from 



