614 EEPOET — 1886. 



merates, tog-ether with others in the Upper Neocomiau of England, have been so. 

 fully described b}' Mr. 'V\'alter Keeping ^ that I need not enter into further details, 

 though I am well aware that the subject is by no means exhausted. 



For a like reason I may pass briefly over the remarkable erratics found in the 

 Cambridge greensand.'- They occasionally slightly exceed a cubic foot in volume, 

 but are generally smaller. Among them are diverse sandstones and giits, probably 

 PalEeozoic, granite, gneiss, various schists, quartzites and slates, besides greenstone,, 

 a very coarse gabbro or hypersthenite, and a compact felstone. I think it highly 

 probable that many of these erratics came from the north, in some cases almost 

 certainlj^ from Scotland, and were transported by ice, though I am not satisfied 

 that any exhibit true glacial striae. In the South of England a boulder of old 

 quartzose rock, perhaps a piece of a coarse quartz-vein, crushed and recemented, 

 has been found by Mr. J. S. Gardner in the gault, and in the chalk we have the 

 well-known cases of the granitic rock and other boulders at Penley, near Croydon, 

 and of coal (Wealden or Jurassic) in Kent.' Mr. Godwin-Austen describes other 

 instances of pebbles in chalk, and I have received two or three small specimens- 

 from Mr. W. Hill, from about the horizon of the Melbourne rock, which, however, 

 have not yet thrown any light on the subject. 



Eoceyie. — Previous writers have called attention to the fact that the sand of the- 

 Thanet, Oldhaven, and Bagshot beds is mainly composed of quartz. This is 

 abimdantly confirmed by my own observations. So far as I have seen, in all these 

 the grains are not, as a rule, conspicuously rounded. It can hardly be doubted that 

 older sandstones or granitoid rocks Ijing to the west have furnished the materials 

 of the Bagshot series, which still has so wide an extension in that direction ; their 

 lithological similarity would lead us to look towards the same quarter for the 

 materials of the more limited Oldhaven and Thanet l)eds. The well-rolled fiint 

 pebbles in the Oldhaven series, and in occasional layers in the Bagshot, suggest the 

 proximity of a shore-line of Upper Cretaceous rocks. 



I have had no opportunity of adding to what has been written on the lithology 

 of the limited Pliocene deposits in England, and, as stated at the outset, have 

 excluded from the scope of this address all beds of later date, which have been so- 

 ably discussed by Mr. Mackintosh, Dr. Crosskey, and many other geologists. 



Principles of Interpi'etation. 



In attempting to interpret the facts which I have enumerated we must bear in- 

 mind the following principles : — 



(1) Pebbles indicate the action either of waves of the sea,'' or of strong currents, 

 marine or fiuviatUe. 



(2) The zone in the sea over which the manufacture of pebbles can be can-ied 

 on is generally a very narrow one. It extends from the high-tide line to the depth 

 usually of a few feet below low-water mark. It is estimated that, as a rule, there 

 is no disturbance of shingle at a greater depth than 20 feet below the latter. It is 

 therefore probable that a thick and very widely extended pebble bed is not the 

 result of wave action. 



(3) The movement of the deeper waters of the sea as a rule is so slight that 

 only the very finest sediment can be affected b)^ it. Now and then great currents 

 like the Gulf Stream, or more locally ' races,' may have sufficient power to transfer 

 pebbles and sand, but instances of this will be exceptional, and confined to rather 

 shallow water. The larger coast currents, however, may transport mud to con- 

 siderable distances, but in directions parallel with the main trend of the shores. 



(4) Except where verj- large rivers discharge their water into the ocean, or \n 



' Geol. Mag. Dec. 2, vol. vii. p. 414. 



^ Sollas and Jukes-Browne {Q. J. G. S. vol. xxix. p. 11). 



^ Godwin- Austen {Q. J. G. S. vol. xvi. p. 327). 



* The waves of lakes also have some rounding effect, but this — except in the case- 

 of very large lakes, such as Lake Superior — is not important ; and such cases are, of 

 course, not of common occurrence. 



