﻿282 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Mar. 6, 



feet. This is the conclusion to which I have come after a very 

 detailed mapping of the ground between the two localities ; and 

 the Geological Survey Map, which will before long be published, 

 will show that the course of the beds, though much interrupted by 

 faults, is consistent only with this theory, — the mass of the Wadhurst 

 Clay being not far below the rock-bed, and a great thickness of sand 

 above the clay that rests upon it. There is about 30 feet of sand 

 that is not rocky, and making only a gentle slope, below the harder 

 bed ; and then comes the Wadhurst Clay. These changes of the strata 

 can be best understood by a glance at the vertical section, fig. 1, 

 p. 276, in which each of the rock-beds is represented by broken lines : 

 their appearance and dying out, and the relationship of the clays 

 above them, will be at once comprehended. 



About the lower beds in this part I have not much to say ; they 

 have changed so little from what they are at Tunbridge Wells. The 

 "Wadhurst Clay is just the same mass of clay and shale with clay- 

 ironstone in it (which stone has been dug a good deal near East 

 Grinstead), and with a little calcareous grit and shelly limestone, — 

 the whole being from 100 to 130 feet thick. The Ashdown Sand 

 does not come much west of Ashdown Forest, where I have before 

 described it : the top of it around some inliers there are on the 

 north-east of Grinstead is not rocky, but of the ordinary thin-bedded 

 stone or else of looser sand ; there is, however, a rock-bed lower in the 

 sands, as well as a subordinate bed of clay ; in general character it is 

 just as before. 



There is a gradual dip of the beds from Ashdown Forest both to 

 the north and to the west, so that the lower beds disappear as we go 

 in either direction ; the Wadhurst Clay skirts round the forest and dips 

 down at about West Hoathly ; while the course of the Grinstead Clay 

 is three miles or so to the west. This bed, however, is again reached 

 by a deep valley, along which the Brighton Railway has been made, 

 exposing the best section of it I have seen. At the cuttings at each 

 end of Balcombe Tunnel the Upper Tunbridge Wells Sand is seen 

 underlain by purple clay which passes downwards into blue shale. 

 More than 40 ft. of this Grinstead Clay can be measured on the north 

 side ; and there is probably about 50 ft. of it on the whole ; for in 

 this cutting the bottom of it is not reached : in the southern one, 

 however, the rock is seen coming from underneath. 



V. Nomenclature of the Hastings Sand. 



1. Dr. ManteWs Nomenclature. — We are now in a position to com- 

 pare what I have related with the accounts of Dr. Mantell, and to 

 discuss the question of what names may be proper for these strata. 

 As I have before said, Dr. Mantell makes out two sands separated 

 by " Tilgate Beds," which he describes as sand, calc-grit, and blue 

 clay, in this descending order. I was at first puzzled to know 

 whether the strata he met with hereabouts were the Tunbridge Wells 

 Sand with the Grinstead Clay in it, or the two thick sands with the 



