382 



GEOLOGY. 



grew upon them. Great masses of coral may sometimes be seen stand- 

 ing erect in the rock precisely as they grew, having entirely escaped 









Fig. 176. Fig. 177. 



Fig. 176. — Diagrammatic section showing the relations of rock formations at 



Niagara Falls. (Gilbert.) 

 Fig. 177. — The Niagara, escarpment. (Gilbert.) 



destruction during their burial in the growing reef. In other instances 

 the coral masses represent remnants only, the greater portions having 

 been broken down and worn away by the waves, or dissolved by the 

 ocean waters, soon after their formation. At various points there are 



Sm 



Fig. 178. — Diagrammatic section showing the relations of various sorts of rock at 



Genesee Falls. (Hall.) 



detached fragments showing various degrees of wear, together with 

 coarse and fine detritus, the product of coral comminution. These 

 combine to make up the mass of the reef-rock. Locally there i§ 



