LOWER AND UPPER GREENSAND OE THE SOUTH OE ENGLAND. 
407 
ridge known as Tilburstow Hill. They are here largely worked for road material, 
and, as we have already seen elsewhere, they consist principally of the debris of 
fossil sponges. To reach the hard beds in the quarries now opened it is necessary 
to remove the overlying beds of loam, fuller’s earth, and dark-green calcareous rock, 
belonging to the Sandgate division, which are about thirty-eight feet in thickness. 
Beneath these are beds of siliceous and cherty rock, with thin intermediate layers of 
sandstone, with glauconite grains and a bed of limestone, in all about eleven feet in 
thickness (3'3 m.), which are so filled with spicular remains that they may be regarded 
as a series of sponge-beds. The chert occurs in layers, from 12 to 14 inches in thick¬ 
ness ('3 to ‘35 m.), of a lenticular form, and it passes upwards and downwards into the 
white porous siliceous rock which has been already described as forming an outer crust 
to it. The sponge-spicules are shown in faint outline in thin microscopic sections of 
the chert, and the spicular canals, which have been infilled with glauconite, are very 
distinct. The siliceous matrix inclosing the spicules appears to have been partly 
deposited in a globular form ; it is now either in the condition of chalcedony or 
of crystalline quartz. 
The porous siliceous rock in these sponge-beds is more developed than the chert ; 
there is one layer of it three feet in thickness. Spicular casts are not clearly shown 
in it, but the silica is of the same character as that in which the moulds of spicules 
are well preserved, and it also passes into true chert. 
The thin intermediate beds of quartz-sand, and glauconite grains, also contain 
sponge-spicules or their casts, and thus indicate a continuous succession of sponge-life 
during their deposition. 
Besides the chert and the porous siliceous rock, there is in this series a bed of 
compact grayish-blue limestone, exceedingly hard and tough, filled with sponge- 
spicules which are now of calcite. This bed is from 12 to 14 inches in thickness 
('3 to '35 m.). A thin microscopic section shows numerous spicules imbedded 
in a matrix of granular calcite (Plate 40, fig. 4). The spicules are nearly entirelv 
composed of clear, crystalline calcite, but some are of silica ; and in many the canals 
are preserved. Their forms and dimensions clearly prove that they belong to the 
[same siliceous sponges as those of the chert and siliceous beds i below the 
i layer of limestone, and it is evident that in this latter bed the silica of the 
i spicules has been nearly entirely dissolved away, and the casts x. , been refilled 
with calcite. Exceptionally, an infilling of glauconite takes place instead of calcite. 
No other organic remains besides the spicules are distinguishable in the limestone. 
Dr. Fitton has given a detailed section of these strata at Tilburstow Hill without 
however noticing the organic nature of the chert. The sections at present exposed 
vary to some extent from those described in the “ Strata below the Chalk.” 
Nut field, Surrey .—Beds of gray sandstone, filled with spicules are exposed at this 
place. A microscopic section of the rock, for which 1 am indebted to Professor 
Judd, F.R.S., shows the spicules partly replaced by glauconite and partly dissolved. 
