298 W. SJ. McGee—Origin and Hade of Normal Fauits. 
Let a homogeneous rigid tract resting on a mobile substra- 
weakness will become faults of normal hade. As in faults of 
the first order, beneath the couche in which pressure equals 
crushing strength fracture will give place to flexure; and, since 
the behavior of any yielding stratum will simulate that of the 
mobile substratum, if there be alternations of rigid and yield- 
ing strata, or if all strata be imperfectly rigid, the throw will 
increase toward the surface. 
Dislocations of this character may be designated Normal 
Faults of the Third Order. The principles involved in their 
development were long ago pointed out by Hopkins,* and are 
now too generally recognized to require thorough analysis. 
Except in the last case the influence of joints in determin- 
ing the regimen of faults may be neglected; for joints are 
known to be superficial, while faults are deep-seate 
Since the movements contemplated in the second case must 
be confined to strata of limited thickness, while in the third 
case the assumption of profound initial fractures or’ planes of 
weakness is unwarranted, the grander faults of the globe must 
be referred to other causes; and since differential radial strain 
is an adequate cause, while the phenomena of profound nor- 
mal faults are collectively such as differential radial strain tends 
to produce, the hypothesis that these faults are due to such 
strains is warranted, and is here enounced. : 
If the hypothesis be valid, it will afford the means of codrdi- 
nating faults and flexures ; it will at once sustain the suggestion 
of Gilbert} that Appalachian flexures and Cordilleran fractures 
are but diverse manifestations of identical movements, and ren- 
der the amount of denudation which the former region has 
suffered roughly determinate ; it will go far toward demonstra- 
ting that, whatever be the condition of its interior, the terres- 
trial crust is—as already indicated by Duttont{—in hydrostatic 
uilibrium; and it will afford a new vantage-point from 
which many of the complex problems of orology and terres- 
trial physics may be approac 
. Salt Lake City, Utah, March 23d, 1883. 
* “ Researches in Physical Geology,” Phil. Trans. Roy. Soc., 1842, i, 53. 
+‘U. 8. Geog. and Geol. Surveys West of the 106th Meridian,” I, ’ 
1875, 62. ne 
t Notice of Fisher's “ Physics of the Earth’s Crust,” this Journal, xxiii, 1882, 288. 
