422 
DR. G. J. HINDE ON BEDS OF SPONGE-REMAINS IN THE 
partly hard, and partly soft and friable. It is filled with sponge-spicules and their 
empty casts, cemented together by chalcedonic silica. Quartz-sand and glauconite 
grains are also present, but no calcite, and only a small amount of mica. In the softer 
portions the spicules are so lightly cemented with the other materials that by gently 
breaking up the accretionary masses they may be obtained in great numbers and with 
their forms perfect. The spicules are chiefly of chalcedonic silica ; some appear to be 
partly of crystalline silica; they are nearly transparent in Canada balsam, and require 
to be mounted for the microscope in glycerine. Spicules are abundant also in the 
unconsolidated sand of the whetstone series of beds, and they may be regarded as 
filled with sponge-remains throughout, and forming a continuous sponge-bed. The 
cementing silica, which renders this material suitable for whetstones, is derived from 
the solution of the spicules, and the chalcedonic silica, which has replaced the calcite of 
the molluscan shells in the same beds, may be attributed to the same source. 
Above the whetstone series of sponge-beds, there are beds of sand and sandstone 
(beds 10, 11, of Mr. Downes), and these are again succeeded by sands with siliceous 
rock and chert, which are also composed of sponge-remains. On the summit of Black- 
clown, near Cullompton, there are beds of cherty debris 5 to 10 feet in thickness, 
imbedded in a clayey matrix. Though much weathered, spicules can yet be recognised 
in some of the masses of chert, and it seems to me highly probable that these beds of 
debris represent the chert series or sponge-beds, which at Warminster, Penzlewood, and 
at Vent nor, form the summit of the upper greensand. 
Though detached spicules are so abundant, entire sponges are now comparatively 
rare in the Blackdown beds. The well-known Stphonia pyriformis (S. tulipa , Zittel), 
figured by Sowerby* in Dr. Fitton’s memoir, is stated by Mr. Downes! to be limited 
to a single layer, and the spicules of this species very seldom occur in the sponge-beds, 
which are principally composed of the acerate and trifid forms belonging to tetracti- 
nellid sponges and of the large spicules of megamorine lithistids. 
Haldon Hills, Devonshire. —The upper portion of these hills, like those at Black- 
down, about twenty miles to the north-east, consists of beds of upper greensand 
age. A section of about 30 feet (8'7 m.) of strata is exposed by the side of 
the high road between Exeter and Dawlish. The beds are principally of quartzitic 
and glauconitic sands, with nodules and bands of chert. The sponge-remains princi¬ 
pally occur in a layer of chert from 8 inches to 1 foot in thickness, and in a bed of 
reddish-brown quartz-sand, 10 feet in thickness, at the base of the section. The sand 
is unconsolidated, and it is in places filled with detached sponge-spicules, which can 
be obtained quite free from the matrix. The spicules are for the most part similar in 
form to those at Blackdown, but they are not so well preserved. The silica of these 
spicules is partly crystalline and partly chalcedonic. 
Axmouth, Beer Head, Devonshire. —I have not had an opportunity of examining 
* “ Strata below tbe Chalk,” ‘ Geological Transactions,’ 2 ser., vol. iv., p. 340, pi. 15a. 
f Quart. Journ. Geo 1 . Soc., vol. xxxviii., p. 81. 
