202 SEE— TEMPERATURE, SECULAR COOLING [April 20, 



earthquakes. Observations show that this depth seldom exceeds 

 twenty miles. Not only is there no extrusion of lavas from great 

 depths at present, but it may also be affirmed with equal certainty 

 that there never has been any such deep-seated extrusion since the 

 earliest geological ages, or even since the epoch when our earth 

 had the maximum surface temperature, before the beginning of 

 encrustation. We may in fact feel sure that no currents were stir- 

 ring at any considerable depth even when the surface was wrapped 

 in flaming fluid, long before encrustation and secular cooling had 

 begun. All the steam and other free vapors once within the planet 

 had already been expelled, and formed about it a dense atmosphere. 

 Even if the earth had two or three times its present volume, a 

 calculation of the corresponding pressures throughout the mass 

 shows that there would be very little convective movement possible 

 at any considerable depth. It seems to be true therefore that con- 

 vection has played a very small part in the arrangement of the inter- 

 nal matter of the globe ; and one may infer that the denser elements 

 could not settle towards the center, except in the outermost layers. 

 And the most probable view of the matter of the interior is that it 

 is a magma of all the elements, the increase of density towards the 

 center being due to pressure, which is sufficient to cause complete 

 interpenetrability of all substances, especially under the high tem- 

 peratures there prevailing. 



§ 4. The Hypothesis of Deep Movements Within the Earth Contra- 

 dicted by Historical and Geological Evidence. 

 It is an observed fact, deduced from the study of world-shaking 

 earthquakes, that movements of this character have never been 

 known to originate at greater depth than thirty, or, at the very out- 

 side, forty miles. It follows therefore that earthquakes arise either 

 in the crust or in the layer just beneath it. Now if deeper move- 

 ments of the globe are in progress, some of them would in all prob- 

 ability have been felt within the historical period ; for they would 

 not be similar to ordinary world-shaking earthquakes, but would be 

 of much more widespread character, the shock being of more nearly 

 equal intensity throughout the world. No such world-wide disturb- 

 ance is recorded in the history of the civilized nations, nor is any- 

 thing transmitted to us by the traditions of barbarous tribes which 



