THE LOWER AND UPPER SILURIAN AGES. 63 



First, we may remark the vast abundance and 

 variety of corals. The polyps, close relatives of the 

 common sea-anemone of our coasts, which build up 

 our modern coral reefs, were represented in the Silu- 

 rian seas by a great number of allied yet different 

 forms, equally effectual in the great work of secreting 

 carbonate of lime in stony masses, and therefore in 



Fig. 9. Fragment of Lower Silurian Limestone, sliced and magnified ten 

 diameters, showing the manner in which it is made np of fragments of corals, 

 crinoids, and shells. (From a paper oil the Microscopic Structure of Canadian 

 limestone, " Canadian Naturalist.") 



the building-up of continents. Let us note some of 

 the differences. In the first place, whereas our modern 

 coral-workers can show us but the topmost pinnacles 

 of their creations, peeping above the surface of the 



