IGNEOUS INTRUSIONS AND VOLCANOES. 249 



The evidence fiirnislied by dikes and other intrusions, as "well 

 as by volcanoes, points to the conclusion that the magmas sup- 

 plied to them are derived from deeply seated sources, but the fact 

 that the material forming intrusions and the products of vol- 

 canoes differ widely among themselves has been cited as evidence 

 that they could not have been derived from a common reservoir. 

 This objection is based on the assumption that the highly heated 

 material forming the earth's interior is homogeneous. It has 

 been argued that, if the material within the surface shell was not 

 homogeneous excepting so far as density increases with pressure 

 or is influenced by the increase of heat with depth an adjustment 

 would be established by the flow of matter from one locality to 

 another. In reply, it may be said, however, that this conclusion 

 is inconsistent with the idea of a solid but potentially plastic inner 

 sphere. From the point of view assumed in this essay, it appears 

 that what may be termed a local flow of the matter comprising 

 the earth's interior would not occur unless there was a local relief 

 or a local increase of pressure. The idea that the earth as a whole 

 is a rigid body is in harmony with the conclusions of emiUent 

 physicists and astronomers, while the assumption of local plas- 

 ticity due to local relief of pressure is consistent with the observed 

 movements of elevation and depression familiar to geologists. 



Returning to the consideration of the passage of the conduit 

 of a volcano through the water- charged portion of the earth's 

 crust, some of the phenomena displayed by dormant volcanoes 

 may perhaps be explained. If the lava in the conduit of a vol- 

 cano cools and hardens below the water-charged zone, the life of 

 the volcano to which the conduit leads may be considered as 

 ended. If the cooling takes place at the surface or within the 

 water-charged layer, steam will continue to be generated below 

 the obstruction, and, if means for its gradual escape are not fur- 

 nished, will ultimately lead to an explosion which may blow away 

 a volcanic mountain. In such an occurrence the main explosion 

 would probably be preceded by a breaking of the rocks and pos- 

 sibly subterranean explosions which would bring temporary relief 

 of pressure. The behavior of many dormant volcanoes and the 

 earthquakes that frequently accompany a renewal of their activ- 

 ity might thus be explained. 



When igneous intrusions enter the water-charged portion of 

 the earth's crust but do not reach the surface, steam is also gen- 

 erated, and may assist such intruded magmas in opening passages 

 for themselves and in elevating the domes that are raised above 

 them. When an intruded magma' meets a large body of subter- 

 ranean water a violent explosion must result, which, when near 

 the surface, would blow away the dome above, leaving a depres- 

 sion of the type of Coon Butte, Arizona, or Lonar Lake, India. 



