86 WILLIAM H. HOBBS 



So soon as we attempt to estimate the lateral migration of a 

 point at the shore due to any given sinking of a definite arc upon 

 the ocean floor, we find that the measure of the movement is 

 probably insufficient to account for the duplication of strata in 

 mountain folds. The landward migration corresponding to a uni- 

 form descent to a depth of one mile within an arc a thousand miles 

 in width, if divided equally between the coasts on either hand, 

 would amount to only about i| miles, or, if concentrated upon one 

 coast, to double that amount.' Account is to be taken, however, 

 of a concomitant change in volume due to a relative elevation of 

 isogeotherms into the descending sector of shell. Sedimentary 

 rocks expand upon the average about t9"oVt2" for each degree 

 Fahrenheit, or 2.75 feet per mile per hundred degrees.^ An arc 

 of a thousand miles should thus be extended, from elevation of 

 temperature in a uniform descent of one mile, a distance of between 

 1,985 and 2,770 feet, according to what figure is taken for the 

 geothermic gradient. If this effect be added to that which is 

 due to shortening of the arc, the expansion of the strata in the 

 arc would be about 3I miles per thousand miles of arc. This 

 figure, though somewhat small, is probably of the right order of 

 magnitude to account for the duplicating of strata in mountain 

 ranges. 



If now we consider the possible effect of a disappearance through 

 depression of the Gondwana continent upon outthrust toward the 

 continent of Asia, we are warranted in assuming for Gondwana 

 Land a fairly high level,, for the reason that even within the tropics 

 there was extensive glaciation. The average depth of the Indian 

 Ocean may be taken as 4,200 meters, and the average descent of 

 some three miles over an arc of 5,000 miles is not improbable. If 

 this outthrust caused landward migration of strata upon the Asiatic 

 shore only, the outthrust northward would correspond to a displace- 

 ment of about 52 miles. This is, however, a maximum figure, and 



' J. D. Dana estimated that within an arc corresponding to a quarter of the entire 

 circumference of the globe, a uniform descent of 8 miles would cause a lateral dis-, 

 placement of 12 miles ("On the Origin of Continents," Am. Jour. Sci. (2), III 

 [1847], 97). 



^ T. Mellard Reade, The Origin of Mountain Ranges, London, 1886, pp. 109-12. 



