506 REPORT—1883. 
usually little more than pebbles. In the descriptive list of the more important of 
these boulders and pebbles given in a paper read before the Geological Society, in 
November 1872, by Messrs. W. J. Sollas and A. J. Jukes-Browne (‘On the In- 
cluded Rock-fragments of the Cambridge Upper Greensand,’ Quarterly Journal of 
the Geological Society, vol. xxix. p. 11), the largest specimen mentioned has the 
dimensions 14 x 12 x 6 inches. 
The boulder now noted measures 12 x 93 x 54 inches, and is therefore amongst 
the largest at present known from this bed. It is somewhat triangular in general 
form, one surface being nearly flat, and is very much rounded and worn. On the 
weathered surface dark, purple, wavy lines appear, generally of the thickness of 
a sheet of writing paper, but sometimes a quarter, or even half an inch thick, alter- 
nating with lighter and thicker bands. Where broken the rock is more uniform in 
colour, the bands varying in shades of purple. Occasionally, where much weathered, 
the lighter bands show a tendency to columnar structure, developed perpendicularly 
to the planes of banding. The material is very hard, and not easily broken. The 
surface of the boulder is worn and smoothed, and in some parts may almost be said 
to be polished. Here and there the softer material of the light-coloured bands has 
been worn into small cavities or depressions, and in other places the lines of banding 
are brought into strong relief by a more uniform wearing away of the softer 
bands. 
As is usually the case with the boulders and fossil remains of the Chloritic 
Marl, this specimen has upon its surface a number of attached plicatulee and other 
small shells, and it bears also two patches of the phosphatic nodules characteristic 
of the bed from which it has been obtained, and even a fragment of the marl itself. 
While the boulder has clearly been subjected to very great wear, and has the 
external appearance usually attributed to the action of ice when found in similar 
boulders of more recent periods, there are upon it no distinct or definite scratches 
or grooves. 
Professor Bonney has kindly examined a fragment from this boulder. The 
material is a very compact quartz-felsite, containing small specks of quartz scattered 
in the matrix, which exhibits a distinct and interesting spherulitic structure. In 
concluding his notes on the boulder, Professor Bonney says:—‘ The microscopic 
structure of the rock differs very decidedly from any specimen which I have 
examined from Charnwood. It differs also from the old rhyolitic rocks of the 
Wrekin and of North Wales. Although it has a certain family likeness to all of 
these, enough to embolden one to suggest that it may have been derived from some 
volcanic mass, now lost to sight, which was active in the latest pre-Cambrian epoch, 
I cannot venture to refer it to any locality known to me in Britain. I have, however, 
no doubt that the pebble described by Mr. Watts’ (‘Geological Magazine,’ vol. viii. 
p. 95) ‘is from the same locality,’ 
Taken alone, no theory as to the prevalence or otherwise of floating ice in the 
sea of the period during which the lower part of the chalk was deposited can 
be founded on this particular boulder. But it at all events supports the already 
existing theory, based on the character of the boulders and pebbles already de- 
scribed from the Chloritic Marl. It has two characteristics of ice-borne erratics :— 
1. It is superficially like boulders recognised as having been transported from distant 
sources by ice, and subjected to the peculiar wear and tear incident to ice-action. 
2. Its material is derived from a parent rock which can under no probable circum- 
stances have existed, at the period of the chalk, within a very considerable distance 
of its recently discovered resting-place. We may therefore fairly, I think, accept 
it as evidence of the probability of the existence of floating ice in the sea of the 
chalk period. 
8. Report on the Fossil Phyllopoda of Paleozoic Rocks. 
See Reports, p. 215. 
9. Fourth Report on the Tertiary Flora of the North of Ireland. 
_ See Reports, p. 209. 
