lxXX PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I9OO, 



borings from Mr. Etheridge and from Prof. Boyd Dawkins are 

 published. Let it suffice to say now that the Hythe Beds seem the 

 most apt to thin out. In like manner details of the Wealden, 

 Jurassic, and other formations will be passed by. 



The Wealden and Purbeck Beds. 



"We have seen how considerable is the rate of thinning of the 

 Lower Greensand underground from its outcrop ; but it is small as 

 compared with that of the much thicker Wealden Beds, with which 

 it is here convenient to associate the Purbeck. From the fact that 

 there are but traces of the Wealden Beds on the north of the London 

 Basin, with little also of the Purbeck Beds, it is evident that these 

 formations must disappear somewhere under the middle part of the 

 Basin. 



But before noticing this thinning more attentively, let us give some 

 passing consideration to what has been already learnt of late by works 

 carried to some depth in the Wealden district itself. The chief 

 result perhaps is that our estimates of the thickness of the various 

 divisions have been shown to be often too small, taking Topley's 

 admirable Survey Memoir on the Weald as our standard, the maximum 

 thicknesses therein assigned having been passed, and that not rarely. 

 This, by the way, is a sign of the excellence of that Memoir, for 

 probably the general tendency of writers is to luxuriate in high 

 figures : temperance is not always the easiest virtue. 



Perhaps the most notable boring from this point of view is that 

 at Penshurst, which, beginning low down in the Series, in the 

 Ashdown Beds, has not passed through the Wealden-Purbeck Series 

 until reaching a depth of perhaps 1500 feet. This boring, too, is 

 an illustration of another point that has been brought out by 

 various borings : that is, the frequent difficulty of fixing divisional 

 planes in this great set of deposits, especially when dealing with 

 specimens brought up from considerable depths. 



The trial-boring at Pluckley is of much interest also. Starting 

 on the Weald Clay, and some way from the top of that formation, 

 it has proved a thickness of no less than 720 feet thereof, whereas 

 the underlying Hastings Beds are only 195 feet thick. I believe 

 that there is also some sign of Purbeck Beds in this boring. 



Returning to the subject of the northerly thinning of the Wealden 

 beds as a whole, at Richmond there is no sign of them, although that 

 boring is not 16 miles from the nearest outcrop of the Weald Clay. 



