Time and Change

by John Burroughs

If the original granite is the father of the rocks, the sea is the mother. In her womb they were gestated and formed. Had not this seesaw of land and ocean taken place, there could have been no continental growth. Every time the land took a bath in the sea, it came up enriched and augmented. Each new layer of rocky strata taken on showed a marked change in color and texture. It was a kind of evolution from that which preceded it. Whether the land always went down, or whether the sea at times came up, by reason of some disturbance of the ocean floors in its abysmal depths, we have no means of knowing. In any case, most of the land has taken a sea bath many times, not all taking the plunge at the same time, but different parts going down in successive geologic ages. The original granite upheavals in British America, and in New York and New England, seem never to have taken this plunge, except an area about Lake Superior which geologists say has gone down four or five times. The Laurentian and Adirondack ranges have never been in pickle in the sea since they first saw the light. In most other parts of the continent, the seesaw between the sea and the land has gone on steadily from the first, and has been the chief means of the upbuilding of the land.

To the slow and oft-repeated labor-throes of the sea we owe the continents. But the sea devours her children. Large areas, probably continental in extent, have gone down and have not yet come up, if they ever will. The great Mississippi Valley was under water and above water time after time during the Palaeozoic period. The last great invasion of the land by the sea, and probably the greatest of all, seems to have been in Cretaceous times, at the end of the Mesozoic period. There were many minor invasions during Tertiary times, but none on so large a scale as this Cretaceous invasion. At this time a large part of North and South America, and of Europe, and parts of Asia and Australia went under the ocean. It was as if the earth had exhaled her breath and let her abdomen fall. The sea united the Gulf of Mexico with the Arctic Ocean, and covered the Prairie and the Gulf States and came up over New Jersey to the foot of the Archaean Highlands. This great marine inundation probably took place several million years ago. It was this visitation of the sea that added the vast chalk beds to England and France. In parts of this country limestone beds five or six thousand feet thick were laid down, as well as extensive chalk beds. The earth seems to have taken another hitch in her girdle during this era. As the land went down, the mountains came up. Most of the great Western mountain-chains were formed during this movement, and the mountains of Mexico were pushed up. The Alps were still under the sea, but the Sierra and the Alleghanies were again lifted.

It is very interesting to me to know that in Colorado charred wood, and even charcoal, have been found in Cretaceous deposits. The fact seems to give a human touch to that long-gone time. It was, of course, long ages before the evolution of man, as man, had taken place, yet such is the power of association, that those charred sticks instantly call him to mind, as if we had come upon the place of his last campfire. At any rate, it is something to know that man, when he did come, did not have to discover or invent fire, but that this element, which has played such a large part in his development and civilization, was here before him, waiting, like so many other things in nature, to be his servant and friend. As Vulcan was everywhere rampant during this age, throwing out enough lava in India alone to put a lava blanket four or five feet thick over the whole surface of the globe, it was probably this fire that charred the wood. It would be interesting to know if these enormous lava-flows always followed the subsidence of some part of the earth's crust. In Cretaceous times both the subsidence and the lava- flows seem to have been worldwide.

IV

We seem to think that the earth has sown all her wild oats, that her riotous youth is far behind her, and that she is now passing into a serene old age. Had we lived during any of the great periods of the past, we might have had the same impression, so tranquil, for the most part, has been the earth's history, so slow and rhythmical have been the beats of the great clock of time. We see this in the homogeneity of the stratified rocks, layer upon layer for thousands of feet as uniform in texture and quality as the goods a modern factory turns out, every yard of it like every other yard. No hitch or break anywhere. The bedding-planes of many kinds of rock occur at as regular intervals as if they had been determined by some kind of machinery. Here, on the formation where I live, there are alternate layers of slate and sandstone, three or four inches thick, for thousands of feet in extent; they succeed each other as regularly as the bricks and mortar in a brick wall, and are quite as homogeneous. What does this mean but that for an incalculable period the processes of erosion and deposition went on as tranquilly as a summer day? There was no strike among the workmen, and no change in the plan of the building, or in the material.

The Silurian limestone, the old red sandstone, the Hamilton flag, the Oneida conglomerate, where I have known them, are as homogeneous as a snowbank, or as the ice on a mountain lake; grain upon grain, all from the same source in each case, and sifted and sorted by the same agents, and the finished product as uniform in color and quality as the output of some great mill.

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.