ESTIMATES AND CAUSES OF CRUSTAL SHORTENING I 5 



weaker beds, are subjected to differential shearing under pres- 

 sure, and if distortion occurs they must be thereby thinned. 



The actual thinning and elongation of layers as a result of 

 folding has been noted by Le Conte 1 as an important phenomenon 

 in the Coast Ranges of California, and by Reade 2 in various 

 mountain ranges. 



According to Gilbert, 3 during the introduction of igneous 

 rocks, which formed the Henry mountains, the pressure of the 

 magma normal to the strata was so great that they were extended 

 laterally by flowage a sufficient amount to cover the domes. In 

 the case of the Holme's arch the linear extension was about 2 

 per cent. 



The amount of thickening or thinning, which any given for- 

 mation or layer undergoes, will of course depend upon many 

 factors, among which attitude, strength, pressure, amount of 

 differential movement or shearing are to be considered. 



As noted by Reade, 4 the attitude of the layers is of the great- 

 est importance. In their initial position the tendency of the 

 pressure is to thicken them. This tendency continues as the 

 layers are tilted, until the average dip is 45 °. As soon as the 

 layers upon the average have a greater inclination than 45 ° 

 (Fig. 1), the average effect of the tangential pressure is unques- 

 tionably to thin the layers, although some members at certain 

 places, and especially at the turns, may be thickened. When it 

 is remembered that in the closely-folded mountains the layers 

 generally have dips greater than 45 °, and as explained later 

 (pp. 16-17) such layers usually turn quickly at the anticlines 

 and synclines, it becomes evident that the thinning of the layers 

 and their consequent elongation, as a result of tangential pres- 



1 On the structure and origin of mountains, with special reference to recent 

 objections to the contractional theory, by Joseph Le Conte : Am. Journ. Sci., Vol. 

 XVI, 1878, pp. 299, 301, 302. 



2 The origin of mountain ranges, by T. Mellard Reade : London, 1886, pp. 

 176, 208, 211. 



3 Geology of the Henry mountains, by G. K. Gilbert : Rept. U. S. Geog. and 

 Geol. Surv. of the Rocky mountain region, 1877, pp. 80-82. 



4 Loc. cit, pp. 216-217. 



