I 



ON THE GEOLOGY OF BARBADOS. 171 



of fine white earths, much resembling soft chalk iu general ap- 

 pearance, with subordinate beds of pink and yellow clay and 

 occasional layers of grey pumiceous sand or silt. 



(b) General Lithological Characters, — Considering the small extent 

 of the area over which they are exposed and the moderate thickness 

 which they attain, the Oceanic Deposits are remarkably variable in 

 composition. The series is roughly divisible into five portions or 

 phases, which blend one into another, but are essentially diff'erent 

 in lithological character. 



The first phase was one of calcareous deposits, and consists 

 mainly of white or cream-coloured earthy limestones, some soft and 

 some hard, which resemble parts of the Lower Chalk of England 

 and France. They break into blocks along joint-planes, which are 

 often coated with black oxide of manganese ; they contain from 

 60 to 80 per cent, of carbonate of lime, are full of Globigerince and 

 other oceanic foraminifera, and have a fine powdery matrix in 

 which coccoliths and certain curious crystalloid bodies are often 

 abundant. 



The central part of the series is essentially siliceous, some beds 

 consisting of 77 per cent, of organic silica with only 0*35 per cent, 

 of calcareous matter. These are the well-known Infusorial or 

 Radiolarian Earths, composed almost entirely of siliceous organisms, 

 — radiolaria, diatoms, and sponge-spicules, the broken debris of 

 these remains forming a matrix in which more perfect specimens 

 are scattered. Associated with these beds are layers of felspathic 

 and pumiceous sand or dust, and some of them are rendered gritty 

 by the intermixture of such material. Other beds are so fine, soft, 

 and slightly consolidated that specimens of them are as light in the 

 hand as lamps of pumice. Their exposed portions are generally 

 white, though below the surface they are often yellowish, drab, 

 pink, or streaked with these colours. 



Above these siliceous earths is a second band or zone of cal- 

 careous material, the quantity of carbonate of lime varying from 

 44 to 80 per cent. These beds are generally white and always 

 contain foraminifera in greater or less abundance, but usually 

 mixed with some siliceous organisms. Manganese is often present 

 on the surfaces of the joint-planes. 



Above the beds last mentioned there is a rapid change into a 

 mass of very fine argillaceous earth, red, pink, yellow, white, or 

 mottled. This earth seems to be analogous to the so-called ' red 

 clay ' of modern oceanic depths, but is remarkable in never con- 

 taining more than a trace of carbonate of lime. A few fragments 

 of radiolaria and sponge-spicules occur in this earth, and it would 

 appear to have been formed at a depth where no calcareous matter 

 could be accumulated ; it often exhibits hollow spaces (up to | inch 

 in diameter) which appear to be the casts of small manganese no- 

 dules, the manganese having been dissolved away and the space 

 being either left empty or filled with loose powdery earth. 



There is only one spot in the island where we can be certain of 

 the occurrence of still higher beds than these red and mottled 



