Vol. 50.] OSSIFEROUS FISSURES NEAR 1GHTHAM. 173 



where the fissures appear to have maintained a free surface-commu- 

 nication through practically the whole of their history, very nearly 

 all the bones are dissolved out, and the lime is redeposited in the 

 interstices of the filling and the adjacent rocks, like veins in ser- 

 pentine. 



The description of the fissures near Tghtham may be taken as a 

 supplement or continuation of the paper by Prof. Prestwich on the 

 drifts around that place. 1 It will, therefore, be unnecessary to re- 

 capitulate the description of the Shode, 2 upon whose banks these 

 fissures occur ; suffice to say that the stream now rises on the lower 

 part of the face of the Chalk escarpment, about 1| mile above 

 Ightham, and after flowing over the Folkestone Beds in a general 

 southerly direction, and receiving a westward branch, pierces the 

 Hythe Beds in a picturesque gorge about 80 feet deep. It then 

 winds round more to the east, and in about a quarter of a mile 

 receives a tributary coming down from the N.E., past Borough 

 Green. This latter stream has also carved out a deep valley, which, 

 in wet weather, still carries water. A road has been cut into and 

 along the old valley, exposing the old sandy brick-earth, the com- 

 position of which is very significant, owing to its similarity to the 

 fissure-deposit. It also carries boulders of Ightham stone, chert, 

 flint, and a few bones and other fossils. 



In about another quarter of a mile the Shode receives a second 

 branch from the N.E., thus leaving the area between the two 

 tributaries as a promontory, which, hy the approach of the two 

 streams to each other and the rise of the Greensand escarpment or 

 counterscarp, is more completely isolated. In this the fissures now 

 to be described occur. The Shode then continues its southerly 

 course through the Plaxtol gap till it joins the Medway. Prof. 

 Prestwich, in his graphic description of this river, 3 gives a map and 

 four transverse sections, which are indispensable in the study of 

 this district. But to fully understand the exact condition of things, 

 attention had better first be centred on a section (see fig. l,p.l7-t)from 

 the Chalk escarpment up the counterscarp to Shingle Hill. Here we 

 see the Gault, estimated at 150 to 200 feet, 4 rising up from below 

 the Chalk on the lower part of the face of the escarpment, stretch- 

 ing over and forming the low part of the Holmesdale Valley ; from 

 beneath this rise the Folkestone Beds, about 110 feet 5 thick ; these 

 follow up the dip-slope of the counterscarp till they reach Bitchet 



1 Quart. Journ. Geol. Soc. vol. xlv. (1889) pp. 270-294. 



2 Mr. Topley calls this stream the Plaxtole brook, op. supra cit. pp. 185, 289. 

 About Plaxtol it is called the Bourne. 



3 Op. supra cit. pi. ix. & p. 272. 



4 In ' The Water-bearing Strata of London,' p. 90, Prof. Prestwich refers to 

 a well at Wrotham which gave 120 feet of Gault. Mr. Topley informs me that 

 the thickness of the Gault in this district is greater than was formerly supposed. 

 A well at Shoreham Place gave 225 feet. He estimates the clay in the line of 

 section at a little over 200 feet. 



5 These figures and measurements are all estimated from sections in the 

 neighbourhood and from the table given by Mr. Topley in his Wealden memoir, 

 plate iii. 



