OF COUAL-LIMESTONE PKOM HAKUADOS. 245 



ai) iucli to an inch in diameter. These show a structure similar to 

 the dead Poriies * from the shore, a specimen of which was sent 

 me for comparison. 



The interstices of the coral wore becoming filled with crystalline 

 calcite, and the network of the coral itself seemed changing- to 

 granular crystalline calcite. In this specimen the minute structure 

 of the angular coral-fragments is not so clear as in the preceding. 



In the second variety the material of the rock consists largely of par- 

 ticles either of an already consolidated mud which appears to be in the 

 condition of the matrix previously described, or of minute portions 

 of Litliothamnion or of coral or shell. In size they vary, some being- 

 minute, others sufficiently large to be described as grains. In shape 

 they are usually angular, but are sometimes more or less rounded, 

 and as a whole they are optically denser than the surrounding 

 matrix. Though generally closely packed, they are not all equally 

 80, and in some parts of the same section they are more separated 

 than in others. 



The matrix is finely granular calcite, but coarser in gram than 

 in the preceding variety, and therefore more transparent. It is 

 hardly possible to say whether this was originally fine mud in 

 which the granular structure has been superinduced by filtration, or 

 whether it is calcite deposited between the interstices of the sepa- 

 rate fragments. 



The Litliothamnion is more abundant in this than in any other 

 variety of the coral-rock ; in some sections little else but its struc- 

 ture is seen, small interstices being filled-up with material similar 

 in character to that of the mass of the rock. The area occupied 

 by it is so large, and there is so little evidence of breakage, this 

 too in a rock which itself consists of the comminuted fragments of an 

 already consolidated deposit, that I believe much of the nullipore 

 occupies still, in relation to the surrounding material, the position 

 in which it grew, and may be considered as being in place. 



There are but few foraminifera ; amongst them may be recognized 

 Ampliistegina. 



Small cavities which occur in this and in all the coral-rocks bear 

 some relation in their size and shape to the fragments of calcareous 

 organisms and other particles embedded in the matrix ; they are 

 probably caused by the solution of some particular fragments which 

 yield to the action of acidified water more readily than others. 

 Some cavities doubtless originate in the actual structure of the 

 fragments of coral, polyzoa, or foraminiferal cells, &c. 



In all the coral-rocks there is evidence that organic calcite, i. e. 

 fragments of calcareous organisms and mud formed by the detrition 

 of such organisms, is more easily soluble than precipitated calcite, 

 and I infer that the cavities often indicate the position occupied by 

 calcareous fragments or mud. The cavities become again filled 



* Dr. Hincle kindly examined sections of these rocks, and pointed out the 

 coral-structure with minute borings ; he considei-ed that the iuchided sponge- 

 branches were without much doxxht Poriies, and he recognized also the structure 

 of Lithofkamnion. 



