146 LIFE ON THE EARTH. 



under this designation in the West Indian Archi- 

 pelago are for the most part not of the same species 

 as those which occur in the Indian and Pacific 

 Oceans, and thus the argument acquires a gene- 

 rality and independence of specific forms and pecu- 

 liarities ; which suits it for application to the extinct 

 races, and the earlier reefs constructed often by dif- 

 ferent genera, in the Silurian, Carboniferous, and 

 Oolitic periods. 



Keeping in mind that light, warmth, and proxi- 

 mity to the surface of pure sea-water, are essentials 

 for the life of the reef-making animals, we shall 

 understand the complete segregation of these re- 

 mains from the great mass of argillaceous and are- 

 naceous sediments in the several formations. The 

 limestones of Wenlock and Aymestry, of Plymouth, of 

 Mendip, Flintshire, Derbyshire, and Yorkshire, all of 

 Palaeozoic ages and full of Coral-beds and bands of 

 corals in place and attitude of growth, appear to be 

 little else than the accumulations of Polypean and 

 Crinoidal reliquiae, augmented by the shells and 

 other exuviae of the sea-animals naturally attracted 

 to the growing calcareous accretion. Each of these 

 limestones indicates an interval of rest in the accu- 

 mulation of sediments, a pause of the depression of 

 the sea-bed 1 . 



1 Memoirs ofMalvern in Memoirs of Geological Survey, n. 1. 



