490 Messrs. Carpenter, Jeffreys, and Thomson [Nov. 18, 



III. — Notes on Specimens of the Bottom collected during the First Cruise 

 of the ' Porcupine 1 in 1869. By David Pokbes, F.E.S. 



Atlantic Mud contained in a small bottle marked " Soundings 

 No. 20, 1443 fathoms." 



A complete analysis of this sample shows its Chemical Composition to 



he as follows : — 



Carbonate of lime 50 # 12 



Alumina * (" soluble in acids ") 1*33 



Sesquioxide of iron (" soluble in acids") .... 2*17 



Silica (in a soluble condition) 5*04 



Fine insoluble gritty sand (rock debris) .... 26*77 



Water 2*90 



Organic matter 4*19 



Chloride of sodium and other soluble salts . . 7*48 



100-00 



If we compare the chemical composition as above with that of ordinary 

 Chalk, which consists all but entirely of carbonate of lime, and seldom 

 contains more than from 2 to 4 per cent, of foreign matter (clay, silica, 

 &c), it will be seen that it differs chiefly in containing so very large an 

 amount of rock-matter in a fine state of division. If we subtract the 

 water, organic matter, and marine salts, which would probably in greatest 

 part be removed before such mud could in process of ages be converted 

 into solid rock, even then the amount of carbonate of lime or pure chalk 

 would not be more than at highest some 60 per cent, of the mass. 



As such deposits must naturally be expected to vary greatly in me- 

 chanical character and chemical composition, it would be premature to 

 generalize as to the actual nature of the deposits now in course of forma- 

 tion in the depths of the Atlantic, before a careful examination had been 

 made of a series of such specimens from different localities. The soluble 

 silica is principally from siliceous organisms. 



[Mr. Hunter's analysis of the Atlantic Mud brought up from the 2435 

 fathoms' dredging, will be found in p. 428]. 



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 



* With phosphoric acid. 



