LOWER AND UPPER GREENSAND OF TPIE SOUTH OF ENGLAND. 
405 
The sponge-spicules present in these beds are exclusively those of siliceous sponges, 
and the large majority are acerate and trifid forms belonging to the Tetractinellidae 
with a few of lithistid and hexactinellid type. 
The exposed sections of the lower greensand round Haslemere are insufficient to 
determine the extent to which the sponge-beds are developed in it, but fragments of 
the beds are shown at or near the surface over a considerable district, and they cover 
the surface of Hindhead and some of the adjoining hills. On the slopes of one of the 
1 fills the sponge-beds are extensively worked for road materials. The sandy strata in 
which they occur have been mapped by the Geological Survey as belonging to the 
lower or Hythe division of the lower greensand, which is estimated by Mr. Topley * 
to attain a thickness of about 300 feet in this district. With one or two exceptions 
the sponge-beds in the lower greensand are limited to the lower division of it. 
In a roadside cutting, about half-way between Haslemere and Godaiming, sponge- 
beds are exposed in the form of layers of siliceous and cherty rock which, as presenting 
a very common type of fossil sponge-beds, may be mentioned in detail. The beds in 
question consist of a central layer of light or dark translucent chert with an upper 
and under layer of yellowish, porous siliceous rock. The upper and under crust of 
rock varies from half an inch to two inches in thickness ('012 to '05 m.), whilst the 
central layer of chert is usually of greater thickness ; the boundary between the chert 
and the outer crust is uneven and irregular, and the one kind of rock gradually passes 
into the other. The outer porous crust consists of a matrix of translucent chalcedonic 
silica, with, at times, an admixture of quartz-sand and glauconite grains. This matrix 
is filled with minute cylindrical or fusiform cavities which are the empty moulds of 
sponge-spicules. In the centre of these cavities there extends occasionally an elongated, 
very slender, cylindrical siliceous rod, which is really the solidified infilling of the 
canal of the spicule. Thus, by a peculiar sequence of conditions, the spicules in this 
portion of the rock have been first imbedded in a siliceous matrix, then their hollow 
canals have been filled with silica, and finally the spicules themselves have been com¬ 
pletely dissolved away, leaving only the casts of their outer form and their solidified 
canals. In the central layer of massive chert, on the other hand, very frequently the 
spicules have disappeared ; but that it was equally as filled with spicules as the porous 
crust is shown by the fact that in microscopic sections faint shadowy outlines of the 
spicules can be detected, and the solidified canals are distinguishable even when other 
traces of the spicules are wanting. The silica derived from the solution of the spicules 
of the outer crust appears, in part at least, to have been re-deposited in the central 
layer in the form of chalcedonic chert, and the spicules of this layer have been thus 
enveloped by the cherty matrix. As the refractive index of the spicules and of the 
matrix is similar, the shadowy appearance of their outlines can be accounted for. In 
some instances there are empty spicular cavities in the chert itself, as well as in the 
outer crust. 
“The Geology of the Weald.” ‘Memoirs of the Geological Survey of England and Wales,’ 1875, 
p. 124. 
3 G 
MDCCCLXXXV. 
