measurable quantity, given only the necessary time. The total thickness of known strata measures but about one-three hundred and twentieth part of the earth's diameter, or, in round numbers, twenty-five miles. Thirty thousand feet of this is quite readily identified as belonging to the old Archaic or Laurentian period, and constitutes the oldest stratified deposit known. Even in this, we find the remains of the Eozoon Canadense, which is now universally acknowledged to be the petrifaction of a foraminiferous living organism with a chambered shell. This means that, at this time, the earth's atmosphere must have been very similar to what it is at the present, and that the temperature of the sea was somewhere between the boiling and the freezing points of water. What time had elapsed since the earth was thrown off by the sun in an incandescent state can only be faintly imagined. At the rate of deposition given for the deepest of ocean deposits, this Archaic period would have taken perhaps thirty-six million years; but inasmuch as the water may have been far warmer then than now, and the rainfall more abundant, and the forces of denudation in all respects more active, this figure may be excessive. The next eighteen thousand feet of strata are easily identified as Lower Silurian, by the Diatoms which occur imbedded in them, and these formations include some of the largest deposits of limestone known. At our rate of calculation, this deposit would require no less than nine and one-half million years, and, in assuming this figure, no account is made of the intervals of time during which no deposit took place, although such periods of inactivity must necessarily have been. The Upper Silurian strata consists of twenty thousand feet, the fossils of which are the lower fishes, and for which we must assign a period of time equal to no less than twenty-five million years, inasmuch as these deposits are limestones and sandstones, or the remains of water-living animals and plants.