Vol.51.] CHEMICAL COMPOSITION OF OCEANIC DEPOSITS. 313 



23. Notes on the Chemical Composition of some Oceanic Deposits. 

 By Prof. J. B. Harrison, M.A., F.Gr.S., and A. J. Jukes- 

 Browne, Esq., B.A., F.G.S. (Read March 6th, 1895.) 



Contents. 



Page 



I. Introduction 313 



II. Analysis of Eed Clay 314 



III. Comparison with, the Coloured Earths of Barbados 320 



IV. The Calcareous Oozes 322 



V. Comparison with Tertiary and Cretaceous Chalk 326 



VI. Microscopical Structure of Oozes 327 



I. Introduction. 



When describing in this Journal the raised and consolidated deep-sea 

 oozes which occur in the island of Barbados, we experienced great 

 difficulty in comparing our analyses of these deposits with those 

 made by Dr. Brazier of the corresponding recent oozes. This was 

 especially the case with the ' red clays ' and, in a less degree, with 

 the other deposits. In the paper above referred to 1 we remarked 

 that ' if the Challenger samples of red clay which contained only 

 small amounts of calcium carbonate and the samples of the Barbados 

 argillaceous earths had been analysed by similar methods, a close 

 resemblance in composition would have been found in them ; and 

 the differences would have been such only as would arise from the 

 action of percolating water containing organic acids upon the Bar- 

 bados deposits, from which the calcium carbonate and the iron per- 

 oxide present in a limonitic form would be leached out.' 



We know that the analyses published in the 'Challenger' Beport on 

 Deep-Sea Deposits were made before Dr. Murray was placed in 

 charge of the ' Challenger ' Office and that he was fully aware of their 

 shortcomings, as testified by his discussions of them in different parts 

 of the Beport. It is especially to be regretted that the analyses of 

 the ' red clays ' were not made on a more satisfactory plan, for the 

 method employed by Dr. Brazier was not calculated to bring out 

 the real mineralogical composition of these deposits. 



Dr. Murray very kindly placed at our disposal a typical sample of 

 ' red clay ' and three samples of Globigerina-ooze, all of them in 

 sufficient bulk for satisfactory analysis. These have been carefully 

 analysed by the same methods as those that were employed in the 

 case of the Barbadian deposits. 



With respect to the Globigerina-oozes, we find that the inaccuracies 

 resulting from Dr. Brazier's methods are at a minimum in the case 

 of those which have more than 80 per cent, of calcium carbonate, 

 but even in these the amount of soluble silica is too high, and the 

 ' loss on ignition ' is unaccountably high ; while in the less calcareous 



1 Quart. Journ. Geol. Soc. vol. xlviii. (1892) p. 191. 



