12 PHYSICAL SCIENCE 



operations still in progress on the earth, or by actual records pre- 

 served in her crust. The earth is thus at once the grandest of labor- 

 atories and the grandest of museums available to man. 



Any summary statement, from a non-professional student, of the 

 advances in geology during the past century, would be hopelessly in- 

 adequate. Such a task could be fitly undertaken only by an expert, 

 or by a corps of them. But out of the impressive array of achieve- 

 ments of this science, two seem to be especially 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 activ- 

 ities, mechanical and biological, of our planet; and a million years 

 is the smallest convenient unit for recording the march of those activ- 

 ities. When one considers the intellectual 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 Uniformitarians, these achieve- 

 ments are seen to be amongst the most important in the annals of 

 science. 



The centrosphere is the terra incognita whose boundaries only 

 are accessible to physical science. It is that part of the earth con- 

 cerning which astronomers, geologists, and physicists have written 

 much, but concerning which, alas! we are still in doubt. Where direct 

 observation is unattainable, speculation is generally easy, but the 

 exclusion of inappropriate hypotheses is, in such cases, generally diffi- 

 cult. Nevertheless, it may be affirmed that the range of possibilities 

 for the state of the centrosphere has been sharply restricted during 

 the past half-century. Whatever may have been the origin of our 

 planet, whether it has evolved from nebular condensation 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 hydrostatic measure by the plastic lithosphere, 

 supplemented by internal pressure below, must determine, it would 

 seem, within narrow limits, the actual distribution of density through- 

 out the centrosphere, regardless of its material composition, of its 

 effective rigidity, or of its potential liquidity. Here, however, we 

 are extending the known properties of matter quite beyond the 

 bounds of experience, or of present possible experiment; and we 

 are again reminded of the unity of our needs by the diversity of our 

 difficulties. 



In his recently published autobiography, Herbert Spencer asserts 

 that at the time of issue of his work on biology (1864) "not one 

 person in ten or more knew the meaning of the word . . . and 



