CONSTITUTION OP CHALK AND FLINT. 
147 
the Southern Hemisphere, if Mr. S. Y. Wood’s view of the 
circumstances be correct.* * * § Professor Prestwich (“ Geological 
Address,” 1871) considers that for a long period the northern 
border of this ocean was continuous, but that at the end of the 
period an opening took place which allowed an Arctic current to 
go southward; and Mr. S. Y. Wood considered cold currents to 
have come from the south.f These so much cooled the sea that 
the molluscous and other animals were much affected by the change, 
and many, such as the Ammonites and their allies, ceased to exist 
therein. 
Extraneous Materials in the Challc. — Even before the Arctic 
opening existed, the northern coasts, probably a little to the S. of 
60° north latitude (Godwin-Austen), had winters at least sufficiently 
cold to form coast-ice, which floated off southwards with rock- 
fragments and pebbles from the shore, to be subsequently dropt 
from the melting ice in the open sea. This would account for the 
occurrence of the “ Purley Boulder,” a large rounded block of granite 
in the Chalk at Purley, near Croydon.J This boulder (felspar and 
quartz with interstices, perhaps Scandinavian) was accompanied by a 
mass of rotten, broken, and rounded augitic greenstone and loose quartz 
sand, all of which must have been held together at first, and then 
precipitated together, in a way which ice alone will account for. 
Other materials foreign to the Chalk are also found in it §—namely 
—•volcanic scoriae, clay-slate (in flint), green schist, quartz (varieties, 
some pebbles), quartzite (131b. 14oz. in weight at Lewes), sandstone 
and quartz-rock (in pieces of 21b. or 3lb., and 141b., scattered over 
one level in Houghton chalk-pit, Sussex), micaceous sandstone, 
spicular chert, greenstone, and soft sandstone; also basalt and 
syenite in Ireland. These are all waterworn to a greater or less 
degree, and were evidently removed from the upper marginal zone 
(shingle-line) to the greater depths. Some were first shifted to 
the submarginal zone, where such pebbles and fragments become 
coated with small parasitic shells, natural to that depth and condition 
(Godwin-Austen), and then carried further down the slopes by 
currents or otherwise. To this list also belong numerous fossils found 
frequently in the Chalk, such as fragments of Sphcerulites and other 
shells; the little corals ( Parasmilia ), which have always broken 
stems, having been forcibly removed from the submarginal zone; 
the Polyzoa , Serpulce , small Ostrece , etc., so frequent on the deserted 
tests of Echinoderms, and such dead shells themselves, though 
others quite perfect were also imbedded where they lived. 
Thus many fossils common in the Chalk, as well as extraneous 
stones, have been drifted from shallower waters nearer the shores. 
Bloating trees and seaweed with attached molluscs and other 
organisms may have carried some things seaward; fishes and large 
* ‘ Phil. Mag.’ 1862, and ‘ Quart. Journ. Geol. Soc.’ vol. xvi, p. 328. 
f ‘ Quart. Journ. Geol. Soc.’ vol. xvi, p. 329. 
% Ibid. vol. xiv, p. 252. 
§ Ibid, vol. xiv, p. 254, etc.; and Dixon’s ‘Geology of Sussex,’ 2nd ed., 
p. 129. 
