MOVEMENTS OF ROCKS, ETC. 795 



In the same way I attribute to this weakening of the rocks by creep 

 the formation of branch canons, the creeping movements working 

 back into the rocks from every new precipice as it is formed*. 



Next, just a word as to the formation of faults. It appears to me 

 that, as reverse faults are admittedly due to horizontal compression, 

 so faults of "normal" hade should be attributed to horizontal 

 extension. Por if, when a fissure is formed in the way I have 

 explained, the rock on one side of the fissure overhangs, there will, 

 on account of the greater weight pressing on the plastic material 

 beneath on this side, be a subsidence of the rock on this relatively 

 to that on the other side of the fissure. The movement will generally 

 go on until, by the shifting which takes place, the fissure is closedt, 

 and, if the spreading or extension is continued for a long period of 

 time, so as to allow the complete plasticity which all rocks ultimately 

 manifest to come into play, Ithink most, if not all, of the peculiarities 

 of this kind of faulting could be readily accounted for. 



A few words in conclusion with reference to the extent of that 

 horizontal compression of the earth's superficial crust which is seen 

 to have been extensively associated with the elevation of mountain 

 ranges, and which reveals itself by greater or lesser folds and con- 

 tortions of the strata J. 



In making the familiar comparison between a bale of cloth folded 

 and puckered by lateral pressure and crumpled, stratified, or lami- 

 nated rocks, it has sometimes been overlooked that, while in the one 

 case, the length of the cloth after it has been puckered is the same 

 as when it lay flat, so that the extent of the compression can be 

 easily estimated from the curves produced, this is not so in the case 

 of folded rocks, as concurrently with the bending of the layers some 

 amount of plastic thickening or thinning takes place. 



The evidence of this partial plasticity is found in difi'erences in 

 the thickness of contorted layers of strata, depending on the direction 

 into which the lines of bedding have been forced. This is well shown 

 in an interesting section figured by Sorby §. 



In weighing the evidence of thickening afforded by such a section, 

 it should moreover be remembered that in the early stages of the 

 deforming process, while the curving was inconsiderable, the con- 

 torted layers must have suff'ered thickening throughout their entire 

 length, and not only at the vertices of the curves, and further that 

 this early stage must have been protracted owing to the resistance 

 to the deformation being greater at first. 



I submit, then, that wherever folds or tiltings and displacements 

 have been produced in stratified rocks by lateral pressure, very great 

 thickening of the strata has taken place, particularly in the early 

 stages of the disturbance, before the puckering became considerable ; 



* S"e ' Tertiary Hist, of the Grand Canon District,' p. 62. 



t The closing of the fissure will, no doubt, be accelerated by the spreading of 

 the rock in which it occurs caused by gravitation. 



J See Dana's ' Geology,' 3rd edition, p. 785. 



§ H. 0. Sorby, "On Origin of Slaty Cleavage," Edin. New Phil. Journ. vol. 

 Iv. 1853, p. 139. 



