CHAP. X.] CONTINUITY OF THE CHALK. ay tty 
premature to generalize as to the actual nature of the deposits 
now in course of formation in the depths of the Atlantic, before 
a careful examination had been made of a series of such speci- 
mens from different localities. The soluble silica is principally 
from silicious organisms. 
As regards the probable origin of the pebbles and gravel 
found in the various dredgings, it will be at once seen, from 
the description, that they consist principally of fragments of 
volcanic rocks and crystalline schists. The former of these have 
in all probability come from Iceland or Jan Mayen; whilst the 
latter, associated as they are with small fragments of grey and 
somewhat altered calcareous rock, would appear to have pro- 
ceeded from the north-west coast of Ireland, where the rocks 
are quite identical in mineral character. The north of Scotland 
and its islands also contain similar rocks; but, without being 
at all positive on this head, I am rather inclined to the opinion 
that they have been derived from Ireland, and not necessarily 
connected with any glacial phenomena, believing that their 
presence may be accounted for by the ordinary action of marine 
currents. 
PEBBLES FROM 1,215 FATHOMS (STATION 28). 
The stones were all subangular, the edges being all more 
or less worn or altogether rounded off. The specimens were 
thirty-eight in number, and, upon examination, were found to 
consist of— 
5 Hornblende schist; the largest of these (which also was the 
largest in size of the entire series) weighed 421 grains ({ 
of an ounce), was extremely compact, and was composed 
of black hornblende, dirty-coloured quartz, and some 
garnet. 
2 Mica schist; quartz with mica, the largest weighing 20 
grains. 
Grey pretty compact limestone, the largest being 7 grains 
in weight. 
2 Fragments (showing the cleavage faces rounded off on 
edges) of orthoclase (potash felspar), evidently derived 
Dia 
Or 
