TRANSACTIONS OF SECTION C. 651 



stream. It would appear worthy of consideration whether this great depression, 

 which thus afi'ects all the crust of the volcano impartially, may not have been 

 caused by the falling in of one of the steam cavities which may be presumed to 

 exist under volcanoes after the lavas have been expelled by the steam pressure. 



This would accord with the observation that sedimentary rocks near volcanoes 

 often dip towards those volcanoes. Mr. Goodcbild has informed the author that 

 the sedimentary roclis round Arthur's Seat are much thicker the nearer they are 

 to that old volcano, as if the ground had slowly sunk while they were being 

 deposited. 



Near Ldn the author was shown a small gia, said to have been formed 

 during an earthquake in February 1885. The crack was of a freshness correspond- 

 ing to such a date, and was only a few inches wide, and so short that it could not 

 bo determined whether it extended beyond one bed of lava. It certainly was not 

 an example of the escape of liquid lava from below a crust, nor of a subsidence 

 over a steam cavity, and its chief interest in this connection is as showing that at 

 least three separate sets of causes are at work in producing the giils of Iceland. 



FRIDAY, AUGUST 10. 



1. A joint discussion with Section 11 on the Plateau Gravels, &c., West Kent, 

 was opened by the following two communications : — • 



(a) On the Geology of the Plateau Implements in Kent} 

 By Professor T. Rupert Jones, F.R.S., F.G.S. 



This subject having been fully treated of by Professor Dr. Prestwich, the 

 requisite references to his various memoirs elucidating the general geology of the 

 local drift-deposits, the geological stages of their formation, and the peculiar flint 

 implements of the plateau were given. He has shown that certain superficial soils 

 on the North Downs between Sevenoaks and Ilochester contain numerous rudely 

 worked flints, discovered by Mr. B. Harrison ; and that these were derived from 

 a gravel, of very great antiquity, originally formed on the side of the old Wealden 

 Hill-range or Mountain, which once rose about 3,000 feet above where Crow- 

 borough and other hills in Sussex now are. Man existed at the time of these 

 gravels, and used the flints for tools. These gravels and the implements left in 

 them were removed by natural agencies, such as rain, rivers, sea, frost, snow, and ice, 

 and distributed by torrential streams on the Chalk slopes (now part of the North 

 Downs) at a lower level on the flanks of the range. 



Tliese rude old flint implements have an ochreous colouring, due to ferruginous 

 gravel whence they came ; and are now found on the plateau, sometimes with 

 limited patches of some of the ochreous flint gravel, together with Tertiary 

 pebbles, less-worn flints, and fragments of Lower Greensand, on the red ' clay- 

 with-tiiuts' covering the Chalk. It was shown how desirable systematic excava- 

 tions, to prove the extent and thickness of the implementiferous soil, would be. 



Professor Prestwich's history of the origin of the ancient Wealden Dome, 

 Island, and Hill-ranges, and of the gradual destruction of those uplands, in the 

 course of untold ages, with the resulting formation and removal of successive 

 geological groups of strata, such as the Thanet Sands, Woolwich-and-Reading 

 Beds, London Clay, Lenham Beds, and the old ferruginous gravel with its rude 

 implements above mentioned, was noticed in detail. 



The Diestian or Lenham Beds were formed in the Early Pliocene period ; and 

 the denudation of Holmesdale probably began directly afterwards, at about the 

 time of the Red or the Chillesford Crag in Late Pliocene, or in Post -Pliocene 

 times ; and the old ferruginous gravel had not only been formed, but washed 

 away to a lower level before that time. 



' This paper has been printed in full in Natural Science, October 1894. 



