THE PEESEEVATION OF EOZOON. 97 



detect any traces of fossils. But even in this case 

 careful management of light may reveal indications 

 of structure, as in some specimens of Eozoon described 

 by the writer and Dr. Carpenter. In many cases, 

 Jiowever, even, where the limestones have become 

 jrfectly crystalline, and the cleavage planes cut freely 

 5ross the fossils, these exhibit their forms and minute 

 'ucture in great perfection. This is the case in 

 lany of the Lower Silurian limestones of Canada, as 

 have elsewhere shown.* The gray crystalline 

 ronton, limestone of Montreal, used as a building 

 bone, is an excellent illustration of this. To the 

 naked eye it is a gray marble composed of cleavable 

 crystals ; but when examined in thin slices, it shows 

 its organic fragments in the greatest perfection, and 

 all the minute structures are perfectly marked out 

 by delicate carbonaceous lines. The only exception 

 in this limestone is in the case of the Crinoids, in 

 which the cellular structure is filled with transparent 

 calc-spar, perfectly identical with the original solid 

 matter, so that they appear solid and homogeneous, 

 and can be recognised only by their external forms. 

 The specimen represented in fig. 23, is a mass of 

 Corals, Bryozoa, and Crinoids, and shows these under 

 a low power, as represented in the figure ; but to the 

 naked eye it is merely a gray crystalline limestone. 

 The specimen represented in fig. 24 shows the 

 Laurentian Eozoon in a similar state of preservation. 



* Canadian Naturalist, 1859 ; Microscopic Structure of Canadian 

 Limestones. 



