574 GEOLOGY. ■'■■■ 



been strongly urged by Fisher* and by Dutton.* In view of the appar- 

 ent incompetency of external loss of heat, the possibilities of distor- 

 tion from other causes invite consideration. 



OTHER SOURCES OF DEFORMATION. 



Transfer of internal heat. — It is theoretically possible that defor- 

 mation of the sub-crust may result from the internal transfer of heat 

 without regard to external loss. It has already been shown (p. 539) 

 that under certain possible conditions more heat would flow from the 

 inner parts to higher horizons than would be conveyed through these 

 latter to the surface and there lost, and that, as a result, the tempera- 

 tures of the inner parts might be falling, while those of the outer parts 

 (except the surface) might be rising. With the more conservative 

 coefficient of expansion previously given, a lowering of the average 

 temperature of the inner half of the earth 500° C. and the raising, by 

 transfer, of the outer half to an equal amount would give a lateral 

 thrust of about 83 miles, which is about the order of magnitude thought 

 to be needed. It is not affirmed that this takes place, but some transfer 

 of this kind is among the theoretical possibilities under the accretion 

 hypothesis. The process could not continue indefinitely; but, for aught 

 that can now be affirmed, it may still be in progress. 



Denser aggregation of matter. — As already noted, matter under 

 intense pressure tends to aggregate itself in the forms that give the 

 greatest densit}^ If the earth were built up of heterogeneous matter 

 arranged at haphazard, the material would probably readjust itself 

 more or less, as time went on, into combinations of greater and greater 

 density. This process may be one of the important sources of shrink- 

 age, for an average change of density of 1 percent., affecting the matter 

 of the whole globe, would probably meet all the demands of deformation 

 since the beginning of the Paleozoic period. 



Extravasation of lavas. — It is obvious that if lavas are forced out 



from beneath the crust and spread upon it, a compensating sinking of 



the crust will follow. This, however, is rather a mode than an ulterior 



cause, for a cause must be found for the extrusion of the lavas, and this 



cause may be one of the other agencies recognized, such as a transfer 



of heat, a reorganization of matter, or a change of pressure. The more 



» Physics of the Earth's Crust, Chap. VIII. 

 Penn Monthly, Philadelphia, May, 1876. 



