Page:Popular Science Monthly Volume 8.djvu/179

Rh the metal from its ores. It is of no less importance in our great chemical manufactures; such, for example, as that of alkali and bleaching-powder. And the agriculturist makes large use of lime in increasing the productiveness of many soils which would be otherwise comparatively 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 rocks, fossils 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 fossils 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 together, is itself composed of minute fragments of the same organic forms, mingled, it may be, with entire specimens 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 investigated by the late Sir William Logan, the distinguished geologist who was employed by the Government of Canada to examine the geological 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, which 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 serpentine 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 that a large continuous zoöphytic growth was produced, bearing a strong