STAGES OF THE EARTH'S HISTORY 433 



of the gases now issuing from volcanoes are a part of what was held 

 in the -original planetesimals, and that they are now reaching the 

 atmosphere for the first time. 



(3) The initial volcanic stage. Before the earth grew to any 

 large part of its present mass, the self-compression which arose 

 from its own gravity is thought to have produced sufficient internal 

 heat to have reached the melting points of the common kinds of 

 rock under low pressures, and as this heat crept outward, it would 

 reach rocks at pressures that would permit liquefaction. Recent 

 discoveries have led to the belief that heat arising from radio- 

 activity has also been an agency in developing high internal tem- 

 peratures, and in thus promoting volcanic action. It is not known 

 how the initial stages of the atmosphere and of vulcanism were 

 related to one another in order of time, but later they ran parallel 

 with one another, and volcanic action is believed to have made 

 notable contributions of gas to the atmosphere. 



(4) The initial hydrospheric stage. Water in the form of gas is 

 light and active, and may at first have escaped; but when the earth 

 had attained sufficient size, water vapor was held in the atmos- 

 phere, and when at length the point of saturation was reached, 

 it took the liquid form and initiated the hydrosphere. The source 

 of the water, according to the hypothesis, was the same as that 

 of the atmospheric gases. 



It may be added here that the hypothesis gives a simple ex- 

 planation of the ocean basins and continental protuberances. It 

 is obvious that, because of unequal growth, the surface of the earth 

 might never have been perfectly spheroidal, so that when the accu- 

 mulation of water upon its surface began, it gathered into the 

 depressions. The planetesimal material which afterwards fell into 

 the water was protected from weathering, while the material that 

 fell on the protuberant areas was exposed to weathering, with its 

 attendant lessening of specific gravity. Thus the depressed areas 

 tended toward higher specific gravities, and hence toward still 

 further depression when deforming stresses were brought to bear 

 on them, while the elevated areas tended to grow relatively lighter, 

 and to suffer relative elevation, under the stress of deformative 

 movements. Thus the differentiation of the oceanic basins from 



