403 
DR. G. J. HINDE ON BEDS OF SPONGE-REMAINS IN THE 
only their infilled canals remaining. The grains of quartz-sand have been cemented 
together by the silica derived from the spicules. 
Sevenoaks, Kent .—On the summit of the lower greensand plateau a short distance 
to the south of Sevenoaks, there are beds of chert filled with sponge-remains which 
are extensively worked for road material. The beds belong to the Hythe division, 
and according to Mr. Topley* they extend from Brastead Chart to some three miles 
beyond Sevenoaks Common, and further east the surface of the ground in Great 
Comp and Mailing Woods is also covered with fragments of the same rock. 
In a stone quarry near the high road, about a mile from Sevenoaks, the following 
section was exposed at the time of my visit. 
feet, inches. 
(1.) Chert (sponge-bed). 2 6 
(2.) Yellow and reddish sand. 2 0 
(3.) Chert (sponge-bed).2 0 
(4.) Sand. 1 0 
(5.) Chert (sponge-bed).2 0 
(6.) Sand.0 8 
(7.) Chert (sponge-bed) bottom of quarry 1 6 
1L 8 
The sponge remains appear to be limited to the beds of chert, of which there are 
four exposed, with a total thickness of eight feet (2’4 m.). The chert is of a gray or 
light-brown tint, very hard and brittle. In some cases there is an outer crust of 
light-coloured porous siliceous rock filled with the casts of spicules. In microscopic 
sections of the chert itself numerous spicules can be seen, embedded in a translucent 
matrix of chalcedony, which appears to have been deposited concentrically round 
them. The spicules are also of chalcedony, and their canals are infilled either with 
silica of a darker tint than the walls, or with glauconite. Frequently all traces of the 
spicular walls have disappeared and only the infilled canals remain, and these might 
readily be mistaken for perfect spicules. A few small foraminifera and fragments of 
echinoderms are occasionally present in the chert ; with these exceptions it seems to 
consist of the remains of tetractinellid and lithistid sponges. 
In the higher beds of the ITythe division, shown in the railway cutting at the 
north end of the Sevenoaks tunnel, I did not recognise any well-defined sponge-beds, 
though in some of the upper beds in the section, both in the so-called hassock and in 
the rag, the rock is in places filled with casts of spicules. 
Fossil sponges are also numerous in beds of loose, unconsolidated, yellowish 
quartz-sand, belonging to the Folkestone or uppermost division of the lower green¬ 
sand, of which a section 25 feet in thickness is shown near the Gas Works at 
* “ Geology of the Weald,” p. 110. 
