1 62 Microscopic Structure of Canadian Limestones. 



life was perhaps young on our planet, the quantity of organic 

 materials thus piled up into rock appears to have been as great as 

 at any subsequent time. Of this some of the silurian limestones 

 of Canada, and more especially the "Trenton Limestone," afford 

 good illustrations, to which I desire in the present paper to direct 

 attention; with the object, not of adding to the knowledge of 

 their fossils, which have been so amply and ably illustrated by 

 Prof. Hall and Mr. Billings, but of noticing the manner in which 

 fragments of these fossils have been accumulated and cemented 

 together into great beds of limestone. 



The lowest of the silurian beds of Canada, the Potsdam sand- 

 stone, is wholly arenaceus, though with a few fossil remains. The 

 Calciferous sandstone has a greater quantity of calcareous matter 

 and more numerous fossil remains. The succeeding beds, the 

 Chazy, Birds-eye, Black River and Trenton Limestone, are as a 

 whole, of organic origin, and made up of more or less commi- 

 nuted fragments of shells, corals, and crinoids, occasionally 

 mixed or alternating with deposits of earthy matter. Above these 

 limestones the TJtica Slate consists mainly of muddy or earthy 

 matter, and in the Hudson River group there are frequent alterna- 

 tions of earthy matter with organic limestones. It thus appears 

 that in Canada, the head quarters of lower silurian limestone of 

 animal origin, is in the central members of the group, which, ac- 

 cording to Sir W. E. Logan, have near Montreal a thickness of 

 nearly four hundred yards, though much thinner in the western 

 part of Canada, as for example in Lake Huron, where Mr. Murray 

 estimates their thickness at only one hundred yards. 



Perhaps the most continuous and fossiliferous of all these 

 limestones is that named by the geologists of New York, from 

 an excellent exposure at the waterfall of that name, the Trenton 

 Limestone. It is largely developed in the vicinity of Montreal, 

 and is thus described by the Provincial Geologist, as it occurs in 

 the quarries near the Mile-end road. " In the vicinity of Mon- 

 treal the lower part of the Trenton formation holds massive beds 

 of gray granular limestone, from which a very large amount of 

 the best building material used in the city has been obtained. 

 The quarries opened on them extend obliquely across that portion 

 of the Cote de la Visitation road, which is southward of the 

 Papineau road, their general direction in respect of one another 

 being about North and South. The beds vary in thickness from 

 three inches up to three feet, and present an aggregate of from 



