Page:Popular Science Monthly Volume 44.djvu/165

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fossiliferous strata. The absence of either air or water on the moon has allowed the steep and jagged mountain rims of the deeply depressed lunar craters to remain nearly unchanged from the almost inconceivably remote time, according to this view, when the asteroid bombardment of the moon was completed.

Geology, or the science of the earth's changes and development, deals with the rocks forming the crust of our globe. From their stratigraphic sequence and the successive fossil faunas and floras found in them, the geologist gathers the history of the sedimentation or volcanic eruption of the rocks and the concurrent changes of animal and plant life. Moreover, by reasoning from the physical condition and structure of the rock formations, from volcanic action, earthquakes, and the upheaval of continents and mountain ranges, he conjectures what may be the condition of the deep interior of the earth, through its observed influence upon the crust. Both these phases of the science have yielded estimates of the age of the earth, the former based on the geologic processes of erosion and deposition, the latter on the earth's loss of internal heat, the magnitude and the effects of the oceanic tides, and other conditions whose investigation belongs more specially to the physicist and astronomer. Each takes up the question for the later part of the earth's planetary existence, after it was so far condensed and cooled that it had already attained approximately its present size and was enveloped by a crust which, through many stages of diverse changes, has continued to the present day.

The estimates derived from these two directions of inquiry, however, differ considerably among themselves, and especially it is noticeable that in general the physical and astronomical investigations of the question yield smaller estimates than those drawn from stratigraphic and paleontologic data. Sir William Thomson (now Lord Kelvin) long ago estimated, from his study of the earth's internal heat, its increase from the surface downward, and the rate of its loss by radiation into space, that the time since the consolidation of the surface of the globe has been somewhere between twenty million and four hundred million years, and that most probably this time and all the geologic record must be limited within one hundred million years. Mr. Clarence King, from very careful experiments on the volcanic rock diabase, supposed to represent the average constitution of the earth's crust, when subjected to extremes of heat and pressure, applying his results in the same way as Lord Kelvin, has within the present year published his conclusion that the earth's duration measures only about twenty-four million years. Prof. George H. Darwin computes, from the influence of tidal friction in retarding the earth's rotation, that probably only fifty-seven million years have elapsed since the moon's mass was shed from the revolving molten