J. G. WALLER ON SAND. 55 



been navigable in historic times. But besides these accumulations 

 by the sea-shore, we are well familiar in the great London basin of 

 deposits of this same shingle with intercalated layers of sand, and 

 the gravel, with its ferruginous hue, known to all for its use in our 

 garden walks. This last, the most superficial of such deposits, caps 

 the London clay over a large part of the metropolitan area. There is 

 another earlier, known as the Bagshot sands, of which Hampstead 

 Heath gives us an example easy for examination. Proceeding 

 downwards, we pass through the vast mass of the London clay, and 

 come to the Pebble bed, well represented at Blackheath, and well 

 named for its small rounded pebbles, like marbles of various sizes, 

 and mixed with this is sand. All these pebbles are of chalk flint. 

 Deposits, more or less mixed with sand, succeed these, until we 

 arrive at the " Thanet sands," lying on the chalk. Of what material, 

 then, are all these sands, and wherein derived is the question pro- 

 posed to this Society. 



Of course the prima facie view is that they naturally arise from 

 the attrition of the flint. Nothing is more apparently obvious. 

 Away from the region of the chalk flint, our coast sand is mostly 

 composed, as we might imagine, of the debris of the adjacent cliffs 

 or rocks, or from other prolific but neighbouring sources of supply, 

 such as shells of molluscs, or calcareous particles of the remains of 

 various zoophytes, as, for instance, at Land's End, and other parts of 

 Cornwall. After we pass westwards of the estuary of the Exe, 

 chalk flint is of rare occurrence on our coasts, although an outlier of 

 the chalk debris may be seen west of but close to the Teign. 



Now then we will proceed to see how far this question belongs to 

 us as a Microscopical Society. Let us take a pinch of sand out of a 

 washing down of a road paved with gravel, after storms of rain, and 

 submit the same to the microscope ; or, to be certain in our experi- 

 ments, let us pound up some chalk flint finely. Our examination 

 of it will show us that the flint has a granular appearance, and does 

 not polarize.* Let us now take a piece of quartz and reduce it to 

 powder, and submit this to the microscope, and we find it to be 

 translucent and clear, and it polarizes brilliantly. Moreover, the 

 fracture of the quartz is different from that of the flint. These con- 

 ditions understood, we are now prepared for the problem to be 

 solved, one which belongs to the geologist, if not to the physicist. 



* It would be more correct, perhaps, to say that it does not give any 

 prismatic colours. 



