Vol.51.] THE CHEMICAL COMPOSITION OF OCEANIC DEPOSITS. 319 



quantity of alumina remaining almost constant' (pp. cit. p. 307). The 

 clay is only this palagonite in a still more disintegrated form. 



We are therefore in complete agreement with Dr. Murray's views 

 regarding the origin and formation of the argillaceous material 

 which covers such large areas of the ocean-floor, and we think that 

 our analysis supplies the chemical confirmation of his conclusions 

 which was wanting in those analyses that were before him when 

 writing his Report. 



There remains, however, an interesting ingredient which has not 

 been fully considered by Dr. Murray, and this is the presence of 

 disintegrated but comparatively unaltered acidic pumice or pumiceous 

 sand in the deposit. The peculiarity of this is that it represents a 

 pumice in which the proportion of alkalies is very high, the amount 

 of soda being especially large. Its re-calculated percentage composi- 

 tion is given below in column A. Column B shows the composition 

 of a piece of pumice from the ' Challenger ' collection analysed by 

 Dittmar, after treatment which freed it from its decomposed external 

 portion. 1 An analysis of a pumice from Cotopaxi is given in 

 column C for comparison. 2 



A. B. C. 



Silica 67-84 7Cv41 73-77 



Alumina 15-39 15-53 1083 



Ferric oxide -68 trace 1'80 



Lime 1-37 211 121 



Magnesia 3-52 -40 130 



Potash 4-12 226 3-90 



Soda 705 2-98 4-29 



Moisture — -20 2-85 



9997 99-89 99-95 



It will be noticed that these are all pumices, in which there is 

 little or no iron, and in which the amount of soda exceeds that of 

 potash, the excess of soda in the pumice from the Red Clay being 

 especially large. They seem to belong to lavas which would solidify 

 into keratophyres or soda-felsites, and we give analyses of three 

 keratophyres for comparison, A being one from near Rubeland in 

 the Harz, quoted from Zirkel, ' Lehrb. d. Petrogr.' 2nd ed. 1894, 

 vol. ii. p. 334, B and C soda-felsites from Co. Wicklow, Ireland. 3 



A. B. C. 



Silica 61-67 71-6 77-8 



Alumina 1747 16"9 13-2 



Ferric oxide 137 "3 2 



Ferrous oxide 392 "9 - 7 



Lime "18 2'2 trace 



Magnesia 213 "6 trace 



Potash 3-38 13 2'1 



Soda 8-52 4'7 5-1 



Water and loss by ignition, etc. *90 l'l *6 



99-54 99 6 99-7 



1 ' Challenger ' Reports, Deep-Sea Deposits, p. 457. 



2 Taken from Watt's Dictionary of Chemistry, art. Pumice. 



3 See Hatch, Geol. Mag. 1889, p. 546. 



Q. J. Gr. S. No. 203. 2 x 



