HOW OLD IS THE EARTH? 155 



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 develop- 

 ment, 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 sedi- 

 mentation 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 inves- 

 tigations of the question yield smaller estimates than those drawn 

 from stratigraphic and paleontologic data. Sir William Thom- 

 son (now Lord Kelvin) long ago estimated, from his study of the 

 earth's internal iieat, 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, sup- 

 posed 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 



