248 MR. W. HILL ON THE STRUCTURE 



to give a naviculoid appearance to the whole of the particles. In 

 their longest axis the largest measure "02 millim. diameter, and 

 from this they diminish to minute atoms. 



There are no foraminifera, radiolarians, or coccoliths in the 

 material sent me, nor any of the organic fragments such as occur 

 in the coral-rocks. 



EXPLANATION OF PLATE IX. 



Coral-rocks of Barbados, (x about 15.) 



Fig. 1. Coral-rock. Bannatynes. The angular fragments show the structure 

 of coral ; «, portion of Lithothamnion. 



2. Coral-rock, Castle Grrant. a, Lithothamoiion ; b, cavities. 



3. Coral-i-ock, G-uUy, E. of Endeavour, a, fragments of Lithothamnion ; 



b, spine of an Echinoderm; c, foraminiferal fragment, probably of the 

 genus Amjphistegina ; d, coral fragment ; e, shell fragment. 



4. Coval-sand rock, a, cavities; b, c, cavities apparently refilled with 



calcite, showing outline of original sand-grain. 



5. Amphistegina-rock. a, Amphistegina Lessonii ; b, cavity. 



APPENDIX II. — On the Structure of White Limestone /rom 

 Jajviatca. By William Hill, Esq., F.G.S. 



Five specimens of the white limestone of Jamaica were sent b)' 

 Mr. C. Barrington Brown to Mr. Jukes-Browne, who kindly for- 

 warded them to me for examination and comparison. 



Their structure, when seen in thin sections under the microscope, 

 shows that they are limestones formed by the accumulation of the 

 debris of calcareous organisms, and two at least strongly resemble 

 certain varieties of the Barbados coral-rocks. 



Of these two, one specimen from Mile Gully, Manchester county, 

 consists of angular fragments, similar in size and shape to the 

 pieces which can be recognized as coral in the first division of the 

 Barbados rocks, and these are set in a matrix of what was, in all 

 probability, fine mud, but is now finely granular calcite (see 

 p. 244). The structure of the fragments and the mud is obliterated 

 by the general crystallization of the deposit; this, however, is 

 paralleled in one of the Barbados specimens. 



In the second, also from Mile Gully, Manchester county, the struc- 

 ture is very similar to that of the third division of the Barbados 

 coral-rock. This limestone seems to have been made up originally 

 of rather large organic fragments set in a matrix of fine mud. For 

 the most part the structure of these fragments is lost, their outline 

 only being shown by patches of clear crystalline calcite ; but there 

 can still be identified fragments of the same Litliothamnion as that 

 which is seen in all the coral-rocks of Barbados, and there are also 

 fragments of foraminifera which are probably referable to the 

 genus Amphistegina. The specimen contains moreover many ossicles 

 of a recent star-fish. 



