364 GEOLOGY 



are so small that unless there is a serious error in the estimated rate 

 of thermal loss, or in the coefficients of expansion, cooling would 

 seem to be a very inadequate cause for the shrinkage implied by 

 mountain folds, overthrust faults, and other crustal deformations. 

 This inadequacy has been strongly urged by various students of 

 the problem. 1 In view of the apparent incompetency of external 

 loss of heat, the possibilities of distortion from other causes deserve 

 consideration. 



The transfer of internal heat. Under certain possible condi- 

 tions, more heat would flow from the central parts of the earth to 

 outer zones than would be passed through them to space, as already 

 stated (Fig. 288) . As a result, the temperature of the central parts 

 might be falling while that of sub-surface zones was rising. A 

 lowering of the average temperature of the inner half of the earth 

 500 C. and a raising of the temperature of the outer half an equal 

 amount, would cause a lateral thrust of about 83 miles. Some 

 transfer of this kind is among the theoretical possibilities under the 

 planetesimal hypothesis. The process could not continue indefi- 

 nitely; but computations imply that it may still be in progress. 



The rise of lavas. If lavas are forced out from beneath the 

 surface, a compensating sinking of the outer shell will follow. The 

 great lava-flow of the Deccan is credited with an area of 200,000 

 square miles, and a thickness of 4,000 to 6,000 feet. Vast as this is 

 for a lava-flow, it would form a layer only about 5 feet thick if 

 spread over the whole surface of the globe. The compensatory 

 sinking would cause a lateral thrust, on any great circle, of about 

 31 feet only. It requires a very generous estimate of the lavas 

 poured out between any two great mountain-making periods since 

 the beginning of well-known geological history, to cause a horizontal 

 thrust of any appreciable part of that involved in the folding of a 

 typical mountain system. The case is different, however, if we 

 go back to the Archean times when the amount of extrusion was 

 very large. Very notable distortion may have arisen from the 

 extravasation of the lavas of that era. 



Intrusions of lava rising from lower to higher levels in the 



1 Fisher, Physics of the Earth's Crust, Chap. VIII; and Button, Penn. 

 Monthly, Philadelphia, May, 1876. 



