HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 89 



making these assumptions, it may be held that during the molten state 

 the gases were not fully separated from the molten mass, but that the 

 gases in the molten globe and those in the atmosphere above took on 

 a state of equilibrium relative to one another, the special terms of 

 which are as yet unknown, but which were of such an order that a 

 large part of the future atmosphere and hydrosphere was retained 

 within the body of the earth. When, later, the external atmosphere 

 was consumed by reaction on the surface rocks, the previous equi- 

 librium was destroyed, and any residue of liquid material, charged 

 under the previous condition, would, when freed from confinement, 

 discharge an additional part of its gases. That portion of the mass 

 which solidified under the earlier conditions of equilibrium may also 

 be supposed to have retained much gaseous material, which was freed 

 later under suitable conditions. Under this conception, the primitive 

 atmosphere was much less extensive than that postulated by the 

 standard form of the hypothesis, and much more gaseous material 

 was held in reserve in the earth-body to actuate future vulcanism and 

 to feed the atmosphere and hydrosphere. 



With this basis, the modified view supposes that the formation 

 of the crust was followed by a period of exceptional volcanic activity, 

 and that this volcanic stage was so prolonged and so effective that 

 the primitive crust was deeply buried under volcanic extrusions, and 

 has since been rarely, if at all, exposed, and that the sedimentary series 

 was derived chiefly from the volcanic series. Some of the extruded 

 material is supposed to have constituted lava-flows and some of it 

 massive embossments, while much of it consisted of the fragmental 

 products of the explosive gases. As the series was built up, it became 

 subject to intrusions forming dikes, sills, and batholiths. When the 

 surface temperature had fallen below the boiling-point of water, aqueous 

 action gave rise to sedimentary deposits which mingled, as minor 

 elements, with the dominant volcanic constituents. 



These features fit fairly well the characteristics of the earliest known 

 rocks, as now interpreted. If this supplemental volcanic action be 

 made extensive enough, the difficulty arising from the apparent absence 

 of the rocks of a primitive crust is avoided. 



The hypothesis thus modified is not altogether successful in meet- 

 ing the atmospheric difficulties mentioned above (p. 86), for the primi- 

 tive atmosphere, supplemented by the accessions of this great vol- 



