Vol. 55.] ON THE OCEANIC DEPOSITS OF TRINIDAD. 187 



20 per cent, of the rock. Similar iridescent scales occur in many 

 of the Barbadian earths, but iu smaller quantity. When examined 

 with a high power (g inch) under the microscope, they are seen to 

 be fragments of the frustales of a diatom, the iridescence being 

 caused by the minute delicate markings of its valves. The frag- 

 ments appear to have belonged to the genus Ethmodiscus, which, 

 according to Sir John Murray, is ' quite characteristic of some of the 

 deepest tropical red clays and radiolarian oozes far from land.' ^ 



The results of the observations made in the field, and of the 

 analyses made in the laboratory, combine to show that there is a 

 succession of Oceanic deposits in Trinidad comparable in all respects 

 with that which is found in Barbados. In both islands there is an 

 upward gradation from calcareous marls or chalks, through calcareo- 

 siliceous beds, to purely siliceous material made up of the tests and 

 frustules of radiolaria and diatoms. 



In Trinidad, however, there is a much greater development of 

 the calcareous deposit than in Barbados, while the radiolarian beds 

 are not so thick. These differences are brought out more clearly 

 by the accompanying comparison of the rock-succession in the two 

 islands (fig. 2). Such differences are what might be expected if, as 

 already suggested, one series was accumulated nearer to continental 

 land than the other ; but both are such deposits as are now found in 

 ocean-depths at some distance from land, and the quartz which they 

 contain is in so fine a state of division, and in such small quantity, 

 that its existence is disclosed only by chemical analysis. 



I was not able to make a detailed examination of the so-called 

 argiline-beds of the hill near San Ternando, but I had previously 

 visited the quarries, and from observations on the spot, and from 

 examination of slices cut from the rocks of the hill, I incline to 

 believe that the argiline has been formed by the alteration of 

 the lower Globigerina-vaarle. Under favourable circumstances, per- 

 colating water might remove all the calcareous matter, leaving only 

 the clay and siliceous material; subsequently some of the silica 

 may have been dissolved and redeposited as a siliceous cement. 

 1 agree with Mr. Guppy in regarding the argiline as originally 

 a deep-water deposit similar to the ISi aparima Marls ; but I can see 

 no reason for looking upon it as of earlier date, or as belonging to 

 a different formation. In all probability it is part of the same 

 series, only in an altered condition. 



There remain for consideration the Nariva Series and its re- 

 lation to those beds near San Pernando which have yielded an 

 Eocene or Oligocene fauna, and for which the name San Pernando 

 Beds may conveniently be retained. My opinion that the jN'ariva 

 Clays of the inland area underlie the Naparima (Oceanic) Marls 



^ Challenger 'Rep. 1891, 'Deep-sea Deposits,' p. 31. [It appears, however, 

 that this form has been found in the Arafura Sea and other localities not very 

 far from land ; see the discussion on this paper, p. 189.] 



