298 PROE. J. PRESTWICH ON THE RAISED 



with those that occur in Cornwall ; another specimen (Ko. 38) 

 had a certain resemblance to some of the gabhros from Guernsey ; 

 while one specimen (No. 28) " had the aspect of a very ancient 

 Archaean gneiss " resembling the Hebridean gneiss of Scotland. 



It would appear therefore that none of the boulders of igneous 

 and crystalline rocks can be referred with certainty to any of the 

 rocks on the English or French coasts, with the exception possibly 

 of the serpentine, which is not a very characteristic rock. Mr. 

 Hunt thinks that a few specimens may be derived from the Eddystone 

 and Shovel rocks, and from Triassic strata at the sea-bottom ; but 

 this seems uncertain, especially with reference to the Red Sandstone 

 which was found 20 miles south of the Eddystone, and far from the 

 main body of Triassic strata. 



We must consequently look to other areas for the origin of the 

 blocks found off the Devonshire coast. The presence of large un- 

 rolled chalk-flints, and of the block (No. 23) described by Mr. Hunt 

 as having " all the appearance of I^eocomian sandstone," together 

 with the close analogy of the Salcombe group of boulders with those 

 of Hayling Island and Pagham, points to their having, like these 

 others, come from the eastward. 



Upon the whole, it seems to me most probable that this im- 

 portant group of foreign boulders is derived from the crystalline, 

 metamorphic, and Palaeozoic rocks of Norway, though some of 

 them, together with the boulders of igneous rocks, may have come 

 down the Ehine and Elbe from Central Germany, or from the 

 Ardennes by way of the Meuse. Eeneath the alluvial beds of 

 Holland there are sands and gravels with boulders which in all 

 probabilit}^ had that source of origin. The reddish sandstone- 

 blocks so common in Hayling Island, and now recorded by Mr. 

 Hunt off the Devonshire coast, may have come from the shores of 

 Denmark, North Germany, or Heligoland. But further investigation 

 and a comparison with the rocks of those countries — which I am 

 unable now to undertake — are necessary. 



In any case the evidence, so far as it goes, accords best with the 

 supposition that the blocks were carried on ice-floes and bergs from 

 the North Sea westward, through the Straits of Dover and down the 

 English Channel, dropping their larger blocks in mid-channel or in 

 the deep water of bays. It yet remains to be seen how much 

 farther westward t,hey may have passed, 



§ 4. The Mollusca of the Eaised Beaches. 



As a rule the shells are limited to very few species, and consist 

 of the most common littoral forms of our present coasts. In many 

 places none are found, and it is only at Portland, the Thatcher Rock, 

 Barnstaple Bay, and Chilton Trinity, near Bridgwater, that they 

 occur in any numbers. 



But although the species were few, it is evident that the indivi- 

 duals were often numerous, for shell-sand and finely comminuted 

 shells form in places a large proportion of the matrix in which 



