CHAP, x.] CONTINUITY OF THE CHALK. 515 



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 vol- 

 canic 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 Horblende schist ; the largest of these (which also was the 

 largest in size of the entire series) weighed 421 grains (f 

 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. 

 5 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 

 L L 2 



