Then, after a vast interval, there comes a break: something like an end and a new beginning, as if one day of creation were finished and a new one begun. The different formations lie unconformably upon each other, which means revolution of some sort. There has been a strike or a riot in the great mill, or it has lain idle for a long period, and when it has resumed, a different product is the result. Something happened between each two layers. What? Though in remote geological ages the earth-building and earth-shaping forces were undoubtedly more active than they are now, and periods of deformation and upheaval were more frequent, yet had we lived in any of those periods we should probably have found the course of nature, certainly when measured by human generations, as even and tranquil as we find it to-day. The great movements are so slow and gentle, for the most part, that we should not have been aware of them had we been on the spot. Once in a million or a half-million years there may have been terrific earthquakes and volcanic eruptions, such as seem to have taken place in Tertiary time, and at the end of the Palaeozoic period. Yet the vast stretches of time between were evidently times of tranquillity. It is probable that the great glacial winter of Pleistocene times came on as gradually as our own winter, or through a long period of slowly falling temperature, and as it seems to have been many hundred thousand times as long, this preceding period, or great fall, was probably equally long--so long that the whole of recorded human history would form but a small fraction of it. It may easily be, I think, that we are now living in the spring of the great cycle of geologic seasons. The great ice-sheet has withdrawn into the Far North like snowbanks that linger in our wood in late spring, where it still covers Greenland as it once covered this country. When the season of summer is reached, some hundreds of thousands of years hence, it may be that tropical life, both animal and vegetable, will again flourish on the shores of the Arctic Ocean, as it did in Tertiary times. And all this change will come about so quietly and so slowly that nobody will suspect it. That the crust of the earth is becoming more and more stable seems a natural conclusion, but that all folding and shearing and disruption of the strata are at an end, is a conclusion we cannot reach in the face of the theory that the earth is shrinking as it cools.