Walter Keeping—The Upware and Potton Pebble-beds. 417 
I. Chert.—These fragments also not seldom exceed (a) in size, and 
are more distinctly subangular in form, being sometimes rude 
parallelopipeds with rounded sides and angles. Colour pale grey 
to almost black; also red and yellow. Several distinct types occur 
which prove to be of two different ages, namely, Carboniferous and 
Jurassic. The majority, I believe, have come from the Mountain 
Limestone, for we find in them numerous fragments of Crinoid 
ossicles (arm joints, and round-sectioned columns), minute shells, 
Athyris, Polyzoa, large sponge spicules and (?) Chatetes. These are 
white, grey, or black-colonred; opaque or translucent. But others, 
for the most part of reddish yellow colour and opaque, or but 
slightly translucent, are Jurassic, yielding muricated Cidaris spines 
fragments of shells and hinge of Inoceramus ; Pecten, and other, 
Lamellibranchs.! 
m. A pebble in the cabinet of Prof. Bonney, who thus describes 
it :— 
“ Devitrified Pitchstone.—This remarkable specimen was found by 
Miss Forster, of Newnham Hall. In form it was a flattened pebble 
rather compressed on one side, about 32’ by 51” diameter, and 
nearly 13” in greatest thickness. When brought to me it had been 
broken into two, the fresh surfaces showing a perfectly compact 
structure of pale pinkish-buff colour marbled with vein-like mark- 
ings, a few specks of quartz being also visible. It was evident that 
the markings very closely resemble a fluidal structure, and the rock 
a Rhyolite. Hoping to place this beyond doubt, I had a thin section 
cut, and then found that the structure was very similar to that of 
some of the “devitrified Pitchstones” described by Mr. 8. Allport. 
With transmitted light it seems to be a tolerably clear glass, in 
which is a quantity of opaque dust,—red with reflected light, and 
probably Fe,O;,—which is often aggregated in wavy, cloud-like irre- 
gular bands. There are a few larger grains of the same. With 
crossed Nicols the glass breaks up into the usual mosaic of light 
and dark granules of a rather irregular form characteristic of a 
devitrified glass. It has some resemblance to fragments in certain 
of the Charnwood agglomerates, but a still closer to slides cut from 
devitrified pitchstones from the Wrekin, to which (allowing for the 
paler colour) it has to the eye a considerable likeness. It must, 
however, be admitted that macroscopically, and to some extent 
microscopically, the rock resembles a specimen, given to me by Dr. 
Hicks, from Treffgarn (Pembrokeshire), which Mr. T. Davies con- 
siders of sedimentary origin. Still, though the above-described is a 
little anomalous, I think it more likely to be a true igneous rock. I 
have lately found another pebble of the same kind.” 
At Upware the larger pebbles are much rarer than at Potton, only 
a very scanty sprinkling of 2-inch pebbles being seen upon the 
heaps of rejected rubbish. The phosphatic masses themselves are 
1 Jurassic chert occurs in England in the Purbeck and Portland beds of Wiltshire 
and the Isle of Portland. Mr. E. B. Tawney tells me of beds of chert in the Lower 
Lias of Glamorganshire; and in the North of England Mr. J. F. Walker, of 
Sidney College, kindly refers me to such deposits in the Coralline Oolite near Malton. 
DECADE II.—VOL. VII.—NO. IX, 27 
