Microscopic Sructure of Canadian Limestones. 165 



like the overlying dark limestones, is not used for tbe more import- 

 ant building purposes, must have been, when recent, a chalky 

 rock, made up of very minute fragments of shells and corals; but 

 it has been blackened by the carbonization of its organic matter, 

 and hardened by the penetration of a calcareous cement ; still its 

 general structure under the microscope is not dissimilar from that 

 of chalk. It contains multitudes of minute unbroken shells, some 

 of which have much the aspect of foraminifera, as may be seen 

 in Fig. 1 .; but they may possibly be univalve mollusks. I 



Fig. 1. — Earthy Trenton Limestone, Montreal,, (20 diams.) 

 hope, however, by the examination of a larger number of speci- 

 mens, to determine whether minute foraminifera really occur in 

 these ancient beds. 



Pig. 2. — Crystalline Trenton Limestone, Montreal, horizontal section, 



(10 diameters.) 



In the coarse grained variety the materials are somewhat loosely 

 paced, and in a horizontal section like Fig. 2, may appear quite 



