ox THE GEOLOGY OF BARBADOS. 179 



in his paper on the ' So-called Crystalloids of the Chalk,' ^ describes 

 a form somewhat analogous to those of the Barbados deposits, and 

 Ehrenberg figures as ' Krystaldrusen ' from the Chalk of Upper 

 Egypt and Lebanon forms which closely resemble, if they are not 

 identical with, the rosette-shaped discs described above.^ 



These stellate bodies do not appear to retain their individuality 

 so long as the ordinary coccolith, but are apt to become obscured 

 by the deposition of calcite. In those which have but few rays this 

 seems to commence between each ray, giving the edges at first a 

 ragged appearance, and finally, as in the case of the Plumtree Gully 

 specimen (p. 217), obliterating the crystalloid altogether. 



Coccoliths also show an augmentation of their mass by the 

 deposition upon them of calcite, but it seems to form along the 

 strise or ribs which ornament the rim. Then, while coccoliths give 

 a black cross when viewed with polarized light, the stellate bodies 

 vary under its action, in some cases remaining neutral while in 

 others there is a decided reaction, a difference which seems to occur 

 in all forms. 



Both are destroyed by treatment with a weak solution of acid. 



(b) Siliceous RocJcs. — Turning now to the siliceous deposits, the 

 finest portion of these maybe recognized as the comminuted remains 

 of radiolarians, diatoms, and sponge-spicules. As these remains 

 embrace a vast variety of forms it is impossible to individualize the 

 fragments, though many bear structural evidence of their derivation. 

 It is interesting to note that coccoliths and stellate crystalloids are 

 entirely absent in purely radiolarian deposits. 



In those rocks which are partly calcareous and partly siliceous 

 the finest material is a mixture of calcareous forms and particles 

 with the debris of siliceous organisms. 



Comparison with the English Chalk. — We specially 

 desired to have Mr. Hill's opinion on this point. He finds that the 

 calcareous and calcareo-siliceous earths of Barbados may be com- 

 pared with the Grey, Middle, and Upper Chalk of England, which 

 they resemble in having a matrix of very fine calcareous material 

 (many of the minuter particles being of a definite and similar 

 shape), in which are distributed numerous foraminiferal tests and 

 cells. The two deposits are the result of an accumulation of similar 

 organic material and exhibit the same lithological peculiarities, so 

 that the name Chalk is equally applicable to both, but no Barbados 

 Chalk can be said to agree specimen for specimen with that from 

 any particular horizon in England. 



Examined and compared by means of thin sections, Barbados 

 Chalk differs from English Chalk as follows : — 1, in the entire 

 absence of the shelly fragments of molluscs and echinoderms ; 

 2, in the presence of recognizable radiolarian remains ; 3, in the 

 greater thickness of the cell-walls of the foraminiferal tests and in 

 the clearer details of their minute structure ; 4, in the greater 

 size and abundance of the coccoliths and crystalloids; 5, in the 



1 Ann. & Mag. Nat. Hist. vol. viii. (1861) p. 197, figs. 3 & 4. 



2 Ehrenberg, ' Mikro-geol.' pi. xxiv. figs. 66, 67 a, pi. xxv. figs. 12-15 B. 



