Microscopic Structure of Canadian Limest07ies. 165 



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■sight to twelve feet. In successive quarries, from the one to the 

 other of which the beds can be traced with considerable certainty, 

 individual beds appear occasionally to change in thickness, a 

 massive one gradually dividing in the strike into two or more? 

 or several thin layers uniting into a solid mass. Slight changes 

 in the color also occur, giving shades of lighter and darker 

 gray." 



This gray granular crystalline stone, the texture of which we 

 may see by picking up a chip at any mason's shed in Montreal, 

 is wholly an organic rock, consisting of the hard parts of marine 

 animals, in a fragmentary condition. In some specimens, joints of 

 those curious stalked star fishes, the crinoids or stone lilies, pre- 

 dominate. In others a little branching coral, the Monticulipora 

 dendrosa of Mr. Billings, but ranked as a variety of the Chaetetes 

 lycoperdon by Hail, is more plentiful. In others, creatures of 

 higher organization than the true corals, the Polyzoa, have con- 

 tributed countless fragments of a delicate structure, which may 

 often be seen spreading over the limestone in flat branches, marked 

 ■with little holes or cells like perforations of pins, and belonging to 

 the genus Stictopora of Hall,* probably the Ptilodictya of the 

 European Paleontologists. The limestone does not merely con- 

 tain these organisms ; it is made up of them, sometimes entire or 

 in large pieces, but more frequently in minute fragments from one 

 tenth to one hundredth of an inch in size. Its present solid 

 condition is due to clear transparent calc-spar or carbonate of 

 lime, deposited by water in the interstices and cavities of the frag- 

 ments, like the " congealed water" of Bermuda or the stalagmite 

 of limestone caverns. This substance being perfectly crystalline, 

 has given its own character to the mass, which thus breaks like 

 marble with multitudes of shining surfaces. Under the micro- 

 scope, however, the true character of the material becomes at 

 once apparent, and the animal fragments, rendered distinct by the 

 remains of their organic matter in a carbonised condition, are seen 

 immersed in the transparent calc-spar, like pieces of potted meat 

 in animal jelly. 



To prepare the specimens for the microscope, it is necessary only 

 to select thin fragments, polish them smooth on one side, then 

 attach the smooth surface by any transparent cement to glass, and 

 grind down the opposite side until the limestone is reduced to a 



* Especially S. Acuta, 



