BEACHES, ETC., OP THE SOUTH OF ENGLAND. 



29'; 



thin crust of the Purbeck strata, resting upon those of Portland, and 

 consisting of slaty beds of limestone which contain freshwater shells, 

 and include a bed of tough dark-coloured clay in which are nume- 

 rous fragments of silicified coniferous trunks not distinguishable from 

 those of the Isle of Portland." ^ The Sussex fragments may therefore 

 have been derived from the Boulogne coast, or from the island which 

 then probably existed on the site of the Varne and Ridge shoals. 



Between this and the western portion of the Channel no special 

 examination of the sea-bed for erratic blocks has been made. We 

 only know that it is strewn over in places with banks of flint 

 shingle. Mr. A. E,. Hunt has, however, by the use of a trawl, 

 recently collected a considerable number of foreign blocks from 

 the Salcombe fishing-grounds,^ some 20 to 30 miles off the coast 

 of Devonshire. He informs me that they were found over an 

 area extending from east to west for a distance of 62 miles, and 

 from north to south for a distance of 20 miles. The blocks varied 

 from ^ cwt. to 16 cwt. in weight, and some of them measured 2 to 

 3| feet in length. 



The 40 blocks described by Mr. Hunt consist of the following 

 rocks : — 



Fine-grained grey Granite. 



Coarse-grained pinkish Granite. 



Hornblendie Granite. 



Microgranulite. 



Syenite (dark green). 



Serpentine. 



Gabbro (mottled purplish green). 



Diabase (dark green). 



Diorite. 



Basalt. 



Trachyte. 

 Arch£ean Gneiss. 

 A Metamorphic Eock. 

 Quartz Grit, 

 Conglomerate Grit. 

 Eeddish-brown Sandstone. 

 Sandstone (Neocomian ?). 

 Chalk-flints in considerable number, 

 and unrolled, — mauy of large size. 



Several of the blocks were much rounded, while others were 

 eubangular, the igneous rocks being often a good deal altered. 



Mr. Hunt dismisses the hypothesis of adjacent land-ice, owing 

 to the absence of similar blocks between Dartmoor and the sea, 

 and concludes that they were brought there by floating ice. But 

 whence ? Mr. E. B. Tawney, who examined the blocks microsco- 

 pically to see whether they could be referred to either side of the 

 Channel, remarks of the granites that he knew of no locality in 

 Britain from which they could be derived, and that in all the spe- 

 cimens there was an absence of schorl, which is so abundant in some 

 of the Dartmoor granites. The serpentine he pronounced to be 

 precisely like some of the Cornish varieties. Again, in com- 

 paring them with specimens of the Channel Island rocks, he could 

 not find any of the characteristic Guernsey rocks among those 

 dredged. 



Prof. Bonney found only a general resemblance in some of the 

 granites (block No. 35 of Mr. Hunt) and in the diabase (No. 37) 



1 Trans. Geol. Soc, 2nd ser. vol. iv. (1836) p. 326. The italics are mine. 



2 ' On the Submarine Geology of the English Channel off the coast of South 

 Devon,' Trans. Devon Assoc. 1880, 1881, 1883, & 1885. 



x2 



