Notices of Memoirs — L. J. Spencer — On LeadhilUte. 71 



on by gales, is continuous, regular, and constant in direction. It is 

 shown by a number of examples that the travel of shingle is not 

 coincident either with the prevailing or predominant winds, but on 

 a tidal coast the predominant drift is invariably in the same direction 

 as that of the flood tide. The action of the tides in heaping up 

 and drifting material is due to wave action. The rise and fall of the 

 tide on the coast does not consist of a mere vertical rise and fall of 

 the water, but of a continual oscillation. The crest of the tidal 

 wave in the open sea, being in advance of that near the shore, 

 results in an oblique lateral movement along the beach, and the 

 advance of the water being checked by the shallow bed with which. 

 it comes in contact, is reflected back, resulting in a series of small 

 oscillations or waves which break when they reach the low-water 

 line. These oscillations are ever present on the margin of the 

 shore, even when the sea is calmest, and are never absent except 

 when absorbed by larger waves due to gales. These tidal wavelets 

 vary in height from 6 inches to 2 feet, and break on the shore at the 

 rate of from ten to twenty a minute according to the rise of the tide 

 and the slope of the beach. These wavelets, aided by the flood 

 current, lift up and carry forward any coarse sand, loose stones, or 

 other material with which they come in contact, and leave some 

 portion of it stranded at the highest point to which the tide of the day 

 reaches. The wavelets, besides lifting and transporting the shingie, 

 brush upward the whole of the face of the bank, and gradually 

 raise it above the line of high-water. It is shown that, though 

 these waves are small, they by their weight and velocity develop 

 sufficient force to move a large quantity of pebbles. A wave having 

 a height of only a foot from trough to crest, giving a head of 

 6 inches, and containing a volume of water equal to a weight of 

 •142 ton has sufficient kinetic energy to raise 165 lbs. of pebbles 

 a foot high. Allowing the weight of pebbles in water to be 100 lbs. 

 to the cubic foot, each wave, if the whole of its energy be applied to 

 the movement of the material, is capable of raising 660 pebbles 

 2 inches in diameter a foot ; or, with fifteen waves to the minute, 

 9,900 pebbles a minute and 2,376,000 in a single tide, or a total 

 weight of stone of 266-4: tons a foot high. This, however, is beyond 

 the work actually done, as a portion of the energy of the wave is 

 absorbed in friction. The above i-ough approximation of the power 

 of the wavelets is sufficient to show the enormous power that is 

 developed by tidal action day by day on the coast, and the capability 

 of the wavelets due to the tides for building up shingle banks and 

 drifting the pebbles along the beach. 



II. — Leadhillite in Ancient Lead Slags from the Mendip 

 HiLLs.i By L. J. Spencer, M.A., F.G.S., British Museum 

 (Natural History). 



LEAD ores have been worked in the Mendip Hills (East Somerset) 

 ever since the time of the Romans ; but during the present 

 century operations have been chiefly confined to the reworking of the 



1 Read before Sect. C (Geology), British Association, Bristol Meeting, Sepb. 1898. 



