Page:Popular Science Monthly Volume 44.djvu/551

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THE POSITION OF GEOLOGY.

THE POSITION OF GEOLOGY.
By Prof. JOSEPH PRESTWICH, F. R. S.

THE position of geology in this country at the present time, more especially as relates to the later geological periods, is anomalous and possibly without precedent. On one side its advance is barred by the doctrine of uniformity, and on the other side by the teaching of physicists. The former requires that everything should be regulated by a martinet measure of time and change. It asserts that the vast changes on the earth's surface, effected during long geological periods, are to be measured by the rate at which similar hut minor changes are effected in the present day, and that the agencies now modifying the surface have been alike, in every respect, in all past time. It is true that no restriction is placed on the extent of the changes, but such prolonged time is insisted on for their accomplishment as to destroy the value of the concession. Not that time is in itself a difficulty, but a time rate, assumed on very insufficient grounds, is used as a master key, whether or not it fits, to unravel all difficulties. What if it were suggested that the brick-built Pyramid of Hâwara had been laid brick by brick by a single workman? Given time, this would not be beyond the bounds of possibility. But Nature, like the Pharaohs, had greater forces at her command to do the work better and more expeditiously than is admitted by uniformitarians.

, On the other side, physicists would lead us to suppose that those great movements of the earth's crust, with which we are all familiar in the form of high mountain and continental upheavals in the earlier stages of the earth's history, were impossible in those times which more immediately approached our own. They maintain that if the earth is not solid throughout, its outer crust at least must have now attained a thickness estimated to vary from eight hundred to twenty-five hundred miles, and is so rigid that we are forced to believe that for a long preceding period it must have been in a state of comparatively stable equilibrium. This, however, would have rendered the great earth movements, considered by geologists to have continued up to the threshold of our own times, impossible. And to this finding the physicists would have geological speculations conform. At the same time, judging, among other reasons, from the rate of cooling of hot solid bodies, they would assign a much shorter term to the earth's history since it became habitable than is compatible with the views of the uniformitarian school of geologists. The one side counts in round numbers upon some three hundred million years; the other sees no reason to go beyond fifteen to twenty million years