ON THE GEOLOGY OF BAHBADOS. 191 



The most noticeable difference in all the samples is the large 

 quantity, varying from 11*03 to 32-60 per cent., of silica " soluble 

 in HCl " (soluble in alkalies after treatment with hydrochloric acid ?) 

 present in all the ' Challenger ' samples as compared with the small 

 proportions (from 3*87 to 5*76 per cent.) found in the Barbados 

 deposits. As, however, Dr. Murray, in his table given on p. 197 

 of the Report, showing the average composition of the ' Challenger ' 

 samples of red clay, places the siliceous organisms present at only 

 2-39 per cent., and as in addition the amounts of alumina shown in 

 the table of analyses as being " soluble in HCl " are in the majority 

 of cases in very high proportion to the total amounts present in the 

 samples, it is evident that the analyses do not represent with 

 accuracy the original proportions of colloid silica present in the 

 samples. They give instead the amounts originally present, together 

 with those set free from the silicates of the clays by the action of 

 strong and probably boiling hydrochloric acid. Again, whilst in 

 the Barbados argillaceous earths the alkalies potash and soda are 

 present in proportions varying from '81 to 2*73 per cent., these 

 usual and important constituents of clay have not been estimated in 

 the ' Challenger ' samples. We consider 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 that the dif- 

 ferences would have been such only as would arise from the action 

 of percolating water containing organic acids upon the Barbados 

 deposits, from which the calcium carbonate and the iron peroxide 

 present in a limonitic form would be leached out. 



Seeing that clays similar in character to these argillaceous earths 

 occur as terrigenous deposits, it is necessary to consider certain 

 other points about them in addition to their general resemblance to 

 the ' Challenger ' samples. Pirst, therefore, ' clay ' present in 

 samples of foraminiferal and radiolarian deposits was separated by 

 elutriation, and, after separation of the colloid silica from it, was 

 found to be similar in nature and in constitution to the argillaceous 

 earths. Again, if the argillaceous earths and the clays present in 

 the other Oceanic Deposits were of terrigenous origin, we should 

 expect them to correspond in composition with the clays of the 

 South American littoral, with those of the underlying Scotland 

 Group which are undoubtedly terrigenous, and with the clay present 

 as an impurity in the coral-rocks of the island. But such is not the 

 case. The clays of the South American littoral, i. e. those of British 

 Guiana derived from the waters of the Orinoco and others of the great 

 South American rivers, invariably contain considerable quantities 

 of quartz, never, as far as our experience goes, less than 15 °/q, and 

 frequently far higher amounts. This quartz is in a state of re- 

 markably fine subdivision, and is not separable from the clay 

 by ordinary processes of elutriation. Similar quartz is present in 

 quantity in the clays derived from the waste of the coral-rocks, 

 and in those of the Scotland Beds ; whilst it is present in only 



