Page:Popular Science Monthly Volume 9.djvu/206

186 polar latitudes, and the return, again, of the waters after 10,500 years. In this way, and in no other that I can conceive of, can be fairly explained the constant mixture and alternations of terrestrial and marine relics, all through the fossil-bearing formations, and the hundreds, if not thousands of different and distinct strata which are found lying one above another.

Whoever, even cursorily, studies the phenomena of geology, must be impressed with the enormous length of time it has taken to arrange the terrestrial substructure, and prepare it for the higher forms of life. Even the comparatively recent period of the Bowlder Clay, which laid out the grounds of the present area of civilization, dates back for its commencement, as we have seen, probably 200,000 years. If it might be assumed that the Permian or New Red Sandstone was formed during the next previous period of extraordinary eccentricity, which was 850,000 years ago, then the Devonian or Old Red Sandstone would come in, very appropriately, at the next anterior era of extraordinary focal distance, which occurred 2,500,000 years back. The Carboniferous period, which came between these two, could not have been formed in less than 1,000,000 years, as most geologists concede; and by calculations previously indicated, those sixty Welsh layers of coal, if there are that many, divided off by marine deposits of considerable thickness, would have consumed 1,250,000 years.

The average thickness of all the strata that lie above the Old Red Sandstone is not far from two miles. But this formation is itself, in many places, two miles thick. And the lower Primary systems will add at least ten miles to the vertical measure of the fossil-bearing rocks. It is estimated that "the fossiliferous beds in Great Britain, as a whole, are more than 70,000 feet in thickness;" and many that are there wanting, or nearly so, elsewhere expand into beds of immense depth. There are certainly fifteen miles deep of strata to be accounted for—the slow accretions of the ages—mainly ocean-sediment that has come down from the wear and washings of the solid rocks. It would be by no means a bold assumption to say that 20,000,000 years had elapsed since the eozoön first built its reefs in the warm Laurentian seas.



OOLS with cutting-edges are not only numerous and varied in form, but they are also varied in the purposes for which they are formed, and in the mode of using. Hence no very precise statement of what is generally meant by a "cutting-edge" can well be 