184 THE GEOLOGY OF BARBADOS. 



samples of pumiceous material it was of some importance to deter- 

 mine the amounts of quartz present, and as the relatively large 

 quantities of pumiceous and crystalline silicates present could not 

 be completely decomposed by boiling sulphuric acid, this was done 

 by fusing the finely powdered sample with a considerable excess of 

 microcosmic salt, which decomposed the silicates without appreciably 

 attacking the quartz. The fused mass was dissolved in water, and 

 the residue boiled with sodium carbonate solution and with hydro- 

 chloric acid, when the portion not dissolved consisted only of the 

 quartz present. This method is easily and rapidly executed, and 

 was found to be an excellent one for the treatment of igneous 

 rocks in which quartz may be present in too small proportion for 

 certain identification by microscopic examination only. 



In all cases the final residues of crystalline silica left were 

 examined microscopically, and, if found impure, the treatment with 

 sulphuric acid and alkaline carbonate or with microcosmic salt was 

 repeated. 



For purposes of analysis the rocks of the Oceanic Deposits may be 

 conveniently classed as follows : — the lower and upper foraminiferal 

 beds (chalks), the calcareo-siliceous rocks, the pure radiolarian 

 marls, the argillaceous earths, and the layers of volcanic material. 

 These divisions are purely arbitrary, as the various beds pass by 

 imperceptible gradations from one to another. Using Haeckel's 

 classification,^ we have foraminiferal rocks passing through mixed 

 radiolarian to pure radiolarian rocks, and these again passing through 

 radiolarian clays to red clay. Similar gradual changes in chemical 

 composition, &c., apparently characterized the samples of deep-sea 

 deposits obtained in the ' Challenger ' Expedition. 



In the tables given the samples are arranged in order of geogra- 

 phical position, proceeding from the south northwards. 



(a) The Loiver and Ujoper Foraminiferal Beds — Chalks and Lime- 

 stones. — These beds show a wide range of composition, and are 

 characterized by the colloid silica in them being difiused through 

 the mass, the remains of siliceous organisms seldom occurring. 

 From the analyses it will be noticed that the colloid silica varies in 

 amount from about 1 to 25 per cent. ; the clay from 3 to IB per 

 cent. ; and the calcium carbonate from 45 to 88 per cent., being, as 

 might be expected, highest in the crystalline limestones, where it 

 generally occurs as a cementing material of crystalline calcite ; 

 but in the case of the Cleland Hill rock the cement is siliceous. 

 Manganese peroxide is present in many of the samples, as black 

 streaks and coatings and as minute spherules. Most of them contain 

 small amounts of zeolitic minerals and only minute amounts of 

 quartz. 



} * Challenger ' Eeports, vol. xYiii. part i, Eadiolaria, p. clxix. 



