ON A PIECE OF LIMESTONE. 167 



the metal from its ores. It is of no less importance in our great clicm- 

 ical manufactures ; such, for example, as that of alkali and bleaching- 

 l^owder. And the agriculturist makes large use of lime in increasing 

 the productiveness of many soils which would be otherwise compara- 

 tively barren. 



Now, let us inquire by what agency, and under what circumstances, 

 these vast limestone formations were produced. 



You all know that, in particular beds of your Avonside rocksfos- 

 sils are met with in great abundance, so that any one who looks for 

 them may find stones that seem almost made up of shells, corals, etc. ; 

 but in other beds, some of them of great thickness, scarcely any traces 

 of fossils are found, the whole rock having a uniform sub-crystalline 

 texture. Now, in regard to the first, it is easy to show that the fos- 

 sils are not merely imbedded in the rock, as they are in a sandstone or 

 a clay, but that the rock is really made up of them ; for, when we cut 

 thin slices of such specimens, and examine them with the microscope, 

 we find that the " matrix," or uniting material by which the fossils 

 are held togetlier, is itself composed of minute fragments of the same 

 organic forms, mingled, it may be, with entire si:)ecimens of minuter 

 forms. But what are we to say of the massive beds of sub-crystalline 

 stone, in which no trace of fossils is to be found ? This question we 

 shall be better able to answer, when we have taken a glance at the 

 other limestones which present themselves in different parts of the 

 great geological succession. 



The oldest stratified rocks of which we have any knowledge are 

 those which make up the great Laurentian formation, first investi- 

 gated by the late Sir William Logan, the distinguished geologist who 

 was employed by the Government of Canada to examine the geologi- 

 cal structure of that country. This formation chiefly consists of 

 quartz, hornblende, felspar, and other mineral constituents, without 

 any admixture of lime ; but near its base is a very remarkable stratum 

 of " serpentine limestone," extending over hundreds of square miles, 

 whicli has a distinctly organic structure. It is composed of a series 

 of layers, usually very thin, of carbonate of lime alternating with 

 serpentine (magnesian silicate) ; and the microscopic examination of 

 the calcareous layers first made by Principal Dawson, of Montreal, 

 and afterward extended by myself, has satisfied us that the calcareous 

 layers form a composite fabric of shelly substance, having a regular 

 chambered arrangement, and that the serj^entine takes the place of 

 the original animal which occupied these chambers and formed the 

 shell. The animal resembled, in its extreme simplicity of structure, 

 the minute "jelly-specks" by which the Globigerina-shells that cover 

 the Atlantic sea-bed are even now being formed ; and differed from it 

 only as the animal of a large composite coral mass differs from that 

 of a simple coral, in extending itself indefinitely by budding ; so tliat 

 a large continuous zoophytic growth was produced, bearing a strong 



