192 Correspondence — Mr. W. Keeping. 



coiaiaiEsiE'on^nDEisroiB. 



FOREIGN PEBBLES OF BRITISH BEACHES. 



Sir, — It is no doubt well known to Mr. Birds as well as many 

 other observers, that the foreign pebbles, desci'ibed by him^ from 

 the Brighton and St. Leonard Beaches, are not confined to the S.E. 

 and S. coasts of England, they being indeed far more abundant 

 in some places on the opposite coast of Britain. 



Aberystwyth, in the centre of Cardigan Bay, is, like Brighton, 

 celebrated for the " pebble " riches of its beach, which afibrd em- 

 ployment to a large number of lapidaries in their cutting and polish- 

 ing. Now these "pebbles" are all of them foreign to the district, 

 and many of them are not even British in origin. Flint agates and 

 " onyx" are not uncommon, and jasper is abundant. Besides these 

 there are large numbers of other interesting strangers, many of 

 them igneous rocks, including granites and quartz-felsites in many 

 varieties, both pink and grey ; orthoclase-felsite, porphyrites, 

 basalt, and serpentine, and volcanic agglomerate ; also numerous 

 sedimentary and metamorphic rocks. 



So abundant are these foreign rocks that in some of the small 

 "Welsh bays they are decidedly more conspicuous than the local 

 stones, and handfuls may be gathered in a square yard. 



As to the origin of these pebbles I quite agree with Mr. Birds, 

 that they are washed up from deposits now covered by the sea. I 

 am not, however, able to see how these facts can determine for us 

 the distribution of any vast ice-sheets such as Dr. Croll has described. 

 To settle this question we must find whether the Boulder-clay was a 

 true Till^a land-ice product, or only a marine Boulder-clay, stored 

 with pebbles dropped from melting icebergs ; and this cannot be 

 settled by reference to the pebbles found on the beaches. 



Now, in the case of the foreign stones of the Welsh shingles, none of 

 them occur in the drifts of the neighbouring country, these drifts 

 being entirely the products of local land-ice ; but I have detected 

 some of them in the drifts of the lowlands of Anglesea. These 

 latter are, however, marine Boulder-clays — laminated deposits like 

 the Norfolk Contorted Drift, and containing delicate marine shells in 

 perfect preservation. 



Therefore, I conclude that the foreign pebbles of the beaches are 

 derived, not from any morainic formation produced by a vast ice- 

 sheet, but from a Boulder-clay, formed as a marine deposit in the 

 Irish Sea at a time when that sea was traversed by icebergs, brought 

 hither by currents from the glaciers of Scotland, and Scandinavia. 



The original homes of many of the rocks are unknown to me. 

 There are many Scotch porphyrites, and a few rocks from the Lleyn 

 peninsula. The flints and some basalts may have come from the 

 North of Ireland. 



W. Keeping. 



The Museum, York. 



1 See Geol. Mag. January, 1881, p. 47. 



