64 THE STOEY OF THE EARTH AND MAN. 



sea in coral reefs and islands, the work of the coral 

 animals of the Silurian has been finished, by these 

 limestones being covered with masses of new sediment 

 consolidated into hard rock, and raised out of the sea 

 to constitute a part of the dry land. In the Silurian 

 limestones we thus have, not merely the coral reefs, 

 but the wide beds of comminuted coral, mixed with 

 the remains of other animals, which are necessarily 

 accumulated in the ocean bed around the reefs and 

 islands. Further, these beds, which we might find 

 loose and unconsolidated in the modern sea, have their 

 fragments closely cemented together in the old lime- 

 stones. The nature of this difference can be well 

 seen by comparing a fragment of modern coral or 

 shell limestone from Bermuda, with a similar fragment 

 of the Trenton limestone, both being sliced for exami- 

 nation under the microscope. The old limestone is black 

 or greyish, the modern one is nearly white, because in 

 the former the organic matter in the animal fragments 

 has been carbonised or converted into coaly and bitu- 

 minous matter. The old limestone is much more 

 dense and compact, partly because its materials have 

 been more closely compressed by superincumbent 

 weight, but chiefly because calcareous matter in solu- 

 tion in water has penetrated all the interstices, and 

 filled them up 'with a deposit of crystalline limestone. 

 In examining a slice, however, under the microscope, 

 it will be seen that the fragments of corals and other 

 organisms are as distinct and well preserved as in the 

 crumbling modern rock, except that they are perfectly 



