226 MOVEMENTS AND DEFORMATIONS 



that unless there is serious error in the estimates, 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 urged strongly 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. 



Shrinkage from denser rearrangement of material already has been referred to 

 (p. 225), and the transfer of heat from deeper to more superficial parts will be 

 discussed in Chapter X. 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 indefinitely; but computations imply that it still 

 may be in progress. 



The rise of lavas. If lavas are forced out from beneath the surface, a com- 

 pensatory 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. This would form a layer 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 since the beginning of well-known geological history to cause a 

 horizontal thrust amounting to any appreciable part of that involved in the folding 

 of a typical mountain system. The case is different, however, if we go back to 

 Archean times when the amount of extrusion was very large. 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 earth would have a 

 dynamic effect similar to that of extrusions, so far as the outer part of the earth 

 is concerned, and the amount of intrusive rock is probably far greater than that 

 of extrusive. 



There are other possible factors in deformation which will not be discussed 

 here. 



References on crustal movements. 



Dana, Manual of Geol., 4th ed., p. 345 et seq.; Willis, The Mechanics of the 

 Appalachian Structures, i3th Ann. Report, U. S. Geol. Surv., Pt. II (1893), pp. 

 211-282; LeConte, Theories of Mountain Origin, Jour. Geol., Vol. I (1893), p. 542; 

 Gilbert, Jour. Geol., Vol. Ill (1895), p. 333, and Bull. Phil. Soc. of Washington, 

 Vol. XIII (1895), p. 31; Van Hise; Estimates and Causes of Crustal Shortening 

 Jour Geol., Vol. VI (1898), U. S. Geol. Surv. (1904), pp. 924-931; A. Geikie, Text- 

 book of Geology, 4th ed., pp. 672-702; Chamberlin and Salisbury, Geologic Proc- 

 esses and their results, Chapter IX; R. T. Chamberlin; The Appalachian folds 

 of Central Pennsylvania, Jour. Geol. Vol. XVIII (1910) pp. 228-251. 



Map work. See Plates CLXV to CLXVII of Professional Paper 60, U. S. 

 Geol. Surv., and Exercise XVII, Interpretation of Topographic Maps. 



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

 Philadelphia, May, 1876. 



