W. SJ. MeGee—Origin and Hade of Normal Faults. 297 
must give place to plastic flexure. The full expression for 
moment of fracture and angle of hade in this case is tan 2= 
(n—b—v cos #m-+icos On)m; and since the several factors may 
be approximately evaluated it follows that the regimen of faults 
formed under given conditions, and conversely the conditions 
giving rise to faults of known regimen, are roughly deter- 
Below the couche of origin the substance of the tract must 
be in the condition of ice in a Brahma press or of lead in a 
pipe machine; and for the purposes of the present discussion, 
e. 
In profound faulting, such as is here considered, minor de- 
partures of the plane of fracture from the plane of maximum 
stress may be neglected, since (1) the irregularities would be 
relatively slight, and since (2) incipient movement would tend 
to eliminate all such irregularities. 
Fractures of the origin and character here contemplated may 
be denominated Normal Faults of the First Order. 
Let a homogeneous rigid tract be subjected to simple ver- 
tical compressive stress due to its own weight. Within the 
limits of its elasticity, resulting vertical compression will be 
relieved by lateral extension; but beyond such limits lateral 
Stress will be in defect, and fracture may occur in each prism o 
the stratum of imminent crushing just as if it were independ- 
ent and unsupported laterally. Now as shown by Hodgkinson,* 
fracture in crushed prisms of crystalline texture occurs diago- 
nally to the direction of stress and separates the prism into 
shing may develop a series of diagonal fractures each bi- 
Secting a prism of height equal to the thickness of the stratum 
* Cited by Rankine, “ Civil Engineering,” 4th ed., 1865, 235. 
Au. Jour. Sct.—Tarap Sunres, Vou. XXVI, No. 154.—Oor., 1883. 
