220 PROF. HARRISON AND ME. JUKES-BROWNE ON [Aug. 1 89 5, 



"We learn from the ' Challenger ' Eeport on Deep-Sea Deposits 

 (p. 349) that fragments of light-coloured acidic pumice and of 

 blackish basaltic pumice were of frequent occurrence in these Pacific 

 dredgings, the most numerous being pebbles of liparitic pumice of a 

 whitish or greyish colour when little altered. Again (p. 297), 

 ' Minute fragments of the different varieties of pumice noted above 

 can be detected in all marine deposits, and in some areas the greater 

 part of a red clay, or of the residue of a calcareous ooze after re- 

 moval of the carbonate of lime by dilute acid, may be made up of 

 minute fragments and splinters of pumice.' 



The presence of this pumiceous sand appears to account for the 

 large percentage of silica in the insoluble residue of Dr. Brazier's 

 analyses, which was also a puzzle to Dr. Murray. Remarking on 

 this, he says : — 'Here too there is an excess of silica . . In certain 

 cases the presence of quartz must be admitted, for the quantity of 

 bases is not sufficiently high, but these grains of quartz do not 

 belong normally to the Red Clay . . . We have seen that special 

 conditions, such as atmospheric currents and glacial phenomena, may 

 serve to account for the presence of quartz in certain regions where 

 Red Clays are in process of formation' (op. at. p. 201). 



It is obvious, however, that an occasional and local cause is not 

 an explanation of an universally high percentage of silica in the 

 insoluble residue of Red Clays. Moreover our analysis, as well as 

 microscopical examination, shows the amount of quartz to be in- 

 finitesimal. When the constituents of Red Clay are properly 

 separated, the difficulty ceases to exist, the palagonitic earth and 

 the pumiceous sand being compounds which will account for all the 

 silica present in the deposit. 1 On reference to Dr. Brazier's analyses 

 of this Red Clay, it will be seen that he estimated the total silica 

 at 59-77 per cent. We find only 54'09, a difference of 5 - 68. 

 Dr. Brazier, however, did not determine the alkalies, which we find 

 to amount to 6- 07, a very close approximation to the difference in 

 the estimates of silica. We conclude, therefore, that Dr. Brazier 

 did not determine the silica directly, but calculated it by difference, 

 and that his amount really includes the alkalies. 



III. Comparison with the Coloured Earths op Baebados. 



In the analyses of the red and yellow earths of Barbados, 

 published in our former paper, the constituents were not separated 

 into groups as in that of the recent Red Clay above given. For 

 the purpose of comparison, therefore, we must arrange the results 

 of the latter in a similar form ; this we have done below in column 



A, placing beside it three analyses of the Barbadian earths, namely 



B, a bright red clay from Castle Grant ; C, a yellow one from Mount 

 Hillaby ; and D, a light red clay from Mount Hillaby : — 



1 Microscopical examination of the pumiceous residue of the Red Clay shows 

 that it also contains a few fragments of felspar and similar minerals, not 

 decomposed. 



