1853.] GUMMING — GLACIAL DEPOSITS, ISLE OF MAN. 211 



impetus necessary for advancing or retreating. And there is so mucli 

 resemblance between these indented tracks, and those produced by the 

 common Shrimp at ebb-tides, that the observer cannot help suspect- 

 ing them to have been produced by this*, — perhaps, the oldest of 

 crustaceans, in its movements along the bottom, or (what is more 

 probable) over the sand at the extreme edge of the waves. 



The smaller marks were of course made by younger individuals of 

 the species, whatever that may have been. But why should the in- 

 dentations be parallel and arranged in transverse curved lines ? 



This question leads to the consideration of the circumstances under 

 which these beds were accumulated, because, if under a sufficient 

 depth of water to be out of reach of the tide, there seems no rea- 

 son why the Hymenocaris should not have advanced in a straight 

 direction ; or, if agitated, why there should be any regularity in the 

 indentations. I have already said, that the alternating coarse and 

 line materials of the sandy and micaceous beds indicate shallow water, 

 a conclusion which is, I think, borne out by the occurrence of patches 

 of drifted sand and broken shells on the strata in some neighbour- 

 ing localities (for instance, in the quarry at the entrance to Y-wern) . 

 I would now go further, and suggest that we have here probable 

 evidence of a flat shore, and of an ebbing tide against which the crea- 

 ture was striving to advance, and by keeping along the water's edge 

 as the wave retired, it would necessarily produce a series of parallel 

 indents along the curved edge. Such au explanation would account 

 for the accurate preservation of all these minute impressions in their 

 original sharpness ; because the surface would be dried before it was 

 again covered by fresh sand. I may remark that these indentations 

 are common in the neighbourhood on similar surfaces, and with 

 annelide tracks accompanying them. But I only met with this in- 

 stance of their regular arrangement. 



January 4, 1854. 



Charles Moore, Esq., Robert Hunt, Esq., R. Hall, Esq., Dr. J. 

 Hobbins, and E. S. Jackson, Esq., A.M., were elected Fellows. 



The following communications were read : — 



1. On the Superior Limits of the Glacial Deposits in the Islk 

 OF Man. By the Rev. J. G. Gumming, M.A., F.G.S., Vice- 

 Principal of King William College, Isle of Man. 



In a paper, read before this Society on the 4th of February, 1846, 

 on the Tertiary Formations of the Isle of Manf, I detailed the position 

 and relations to each other of certain pleistocene accumulations, which 



* This is rendered still more probable by the fact, that the Hymenocaris is 

 found at Tremadoc, but the Olenus has not yet been discovered there. 

 t Quart. Journ. Geol. Soc. vol. ii. p. 31 7 ; nith maps and sections. 



