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SCIENCE. 



[N. S. Vol. XX. No. 509. 



the restricted province of geologists, but 

 they now lay claim to the entire earth, 

 from the center of the eentrosphere to the 

 limits of the atmosphere, and they threaten 

 to invade the region of the astronomers on 

 their way towards the outlying domain of 

 cosmogony. Geology illustrates better 

 than any other science, probably, the wide 

 ramifications and the close interrelations of 

 physical phenomena. There is scarcely a 

 process, a product or a principle in the 

 whole range of physical science, from phys- 

 ics and chemistry up to astronomy and 

 astrophysics, which is not fully illustrated 

 in its uniqueness or in its diversity by 

 actual operations still in progress on the 

 earth, or by actual records preserved in her 

 crust. The earth is thus at once the grand- 

 est of laboratories and the grandest of mu- 

 seums available to man. 



Any summary statement, from a non- 

 professional student, of the advances in 

 geology during the past century, would be 

 hopelessly inadequate. Such a task could 

 be fitly undertaken only by an expert, or 

 by a corps of them. But out of the im- 

 pressive array of achievements of this sci- 

 ence, two seem to be specially worthy of 

 general attention. They are the essential 

 determination of the properties and the 

 role of the lithosphere, and the essential 

 determination of the time scale suitable for 

 measuring the historical succession of ter- 

 restrial events. The lithosphere is the 

 theatre of the principal activities, mechan- 

 ical and biological, of our planet ; and a 

 million years is the smallest convenient 

 unit for recording the march of those ac- 

 tivities. When one considers the intel- 

 lectual as well as the physical obstacles 

 which had to be surmounted, and when one 

 recalls the bitter controversies between the 

 Neptunists and the Vulcanists and between 

 the Catastrophists and the Uniformita- 

 rians, these achievements are seen to be 



amongst the most important in the annals 

 of science. 



The eentrosphere is the terra incognita 

 whose boundaries only are accessible to 

 .physical science. It is that part of the 

 earth concerning which astronomers, geol- 

 ogists and physicists have written much, 

 but concerning which, alas! we are still 

 in doubt. Where direct observation is un- 

 attainable, speculation is generally easy, 

 but the exclusion of inappropriate hypoth- 

 eses is, in such cases, generally difficult. 

 Nevertheless, it may be affirmed that the 

 range of possibilities for the state of the 

 eentrosphere has been sharply restricted 

 during the past half century. Whatever 

 may have been the origin of our planet, 

 whether it has evolved from nebular con- 

 densation or from meteoric accretion ; and 

 whatever may be the distribution of tem- 

 perature within the earth's mass as a 

 whole; it appears certain that pressure is 

 the dominant factor within the nucleus. 

 Pressure from above, supplied in hydro- 

 static measure by the plastic lithosphere, 

 supplemented by internal pressure below, 

 must determine, it would seem, within nar- 

 row limits the actual distribution of density 

 throughout the eentrosphere, regardless of 

 its material composition, of its effective 

 rigidity or of its potential liquidity. Here, 

 however, we are extending the known prop- 

 erties of matter quite beyond the bounds 

 of experience, or of present possible ex- 

 periment; and we are again reminded of 

 the unity of our needs by the diversity of 

 our difficulties. 



In his recently published autobiogTaphy, 

 Herbert Spencer asserts that at the time of 

 issue of his work on biology (1864) 'not 

 one person in ten or more knew the mean- 

 ing of the word * * * ; and among those 

 who knew it, few cared to know anything 

 about the subject' That the attitude of 

 the educated public towards biological sci- 

 ence could have been thus indifferent, if 



