RESULTS OF THE EARTH'S CONTRACTION. 739 



present mean depth of the oceanic areas below the mean level of the continental 

 plateaus is probably about 16,000 feet. The thickness of the layer of liquid rock 

 required to make, a depression of 16,000 feet, by its consolidation, would be about thirty- 

 eight and a quarter miles. But, as contraction has gone on through time, over both 

 continental and oceanic areas, this is the mean excess of depression for the oceanic 

 area. What part of this excess existed when the oceanic depression was first made, 

 there are no facts for satisfactorily deciding. If the coral-island subsidence was due 

 in any considerable part to radial contraction, beneath the central Pacific crust itself, 

 it is probable that the excess has increased, even in Cenozoic time. 



The cooling of one part of the crust before the rest must have been a consequence 

 of there being less vivid heat and violent action in the liquid rock of that part; and 

 this may have been connected with the exterior of the solid nucleus, in that part, being 

 nearer than elsewhere to the outer surface of the sphere, that is, to there being less 

 depth of liquid rock over it to cool. 



7. Results of Contraction. — The differences of level thus early 

 developed in the surface of the sphere had a controlling effect over 

 all the subsequent results of contraction. These results include, — 



1. Flexures of the crust and its strata, fractures, earthquakes. 



2. The evolution of the earth's fundamental features. 



3. Changes in climate. 



And, incidental to these, there were igneous eruptions along frac- 

 tures, consolidation of rocks, metamorphism of strata, making of 

 mineral veins, destructions of life, and other progressing changes in 

 the earth's physical condition. 



II. FLEXURES OF THE CRUST, AND OF STRATA, FRACTURES, 

 EARTHQUAKES. 



1. Flexures, Fractures. 



The sudden production of vapors beneath a portion of the earth's 

 crust is referred to, on a preceding page, as a possible cause of local 

 changes of level in volcanic regions. It is often regarded as a means 

 of making mountains and raising continents. But mountain -chains 

 are heavy, and continents very heavy ; and such vapors, if formed, 

 could at the most only shake the rocky crust. Mountain-chains and 

 continents could not be sustained long on a bed of vapors ; for perma- 

 nent elevation, there must be some mode of holding them up after the 

 uplift. Moreover, there is no reason to believe in the existence of 

 cavities beneath, such as would be required for the spread of the 

 vapors. 



Flexures of the Crust and of Strata. — Lateral pressure, from con- 

 traction, is a force of indefinite power, fully adequate for all the moun- 

 tain-making which has taken place. It acts horizontally, or very 

 nearly so, and therefore in a direction to produce the flexures of the 

 rocks involved in the making of mountains. Its first effect is to cause 

 great upward and downward bendings in the crust, — geanticlinals and 



