sediments beneath them (Pl. I). If he has traveled farther north to the Cabinet Range, Montana, he must have carried with him an undying impression of the 35,000 ft. of the rocky monument, there built up by the Proterozoic seas. [Illustration: LEAFLET 9. PLATE II. COMPARISON OF AN ANCIENT AND A MODERN MARINE ANIMAL OF ALLIED GROUPS. 1, A CAMBRIAN TRILOBITE (AFTER WALCOTT). 2, A MODERN HORSESHOE CRAB.] The present rate of denudation in the Hudson Bay region is one foot in about 47,000 years. How long then, has it taken the seas to lay down these miles of sediments, which are but a small fraction of the whole geologic column? Having considered all this, can we estimate, even approximately, the vast length of time that has elapsed between these periods of sedimentation, the so-called “gaps” or “breaks”—the torn, illegibly written pages of the history of the earth? Indeed, there are moments when all may feel that it is much beyond their comprehension. But, man, by nature, is at once humble and exalted. He is willing to admit his defeat, yet his thirst to conquer new knowledge, to know the truth, is never satiated. The reason for the failure to arrive at an absolute result is not very far to seek. In computing geologic time, one has to calculate that which has elapsed, by some process in nature that takes place in one direction only and that does not change its rate when conditions alter. Whatever the method applied, be it the deposition of sediments, or the gathering of solutions, or the losing of heat by the sun and the earth, its rate of action should be uniform and uninterrupted. It should be independent of the changing conditions of the earth. Uniformity of the rate of action is the criterion for precise calculation. But we know that the past was quite different from the present. Different conditions have existed at different times during the earth’s history. The configuration of the earth, its climate, humidity, temperature and many other factors have varied from time to time and with them the rate of erosion, deposition and solution has either accelerated, diminished or ceased. The doctrine of uniformitarianism cannot be assumed in a changing world, even though our knowledge of the earth of the past can only be gained from a fuller study of the earth of the present.