262 Willis and Hayes — Conditions of 



above the not very deep waters along shore. From the begin- 

 ning of emergence the arch will be subject to degradation and 

 its waste will be deposited in the adjacent synclines. Then, 

 the crown of the arch being removed, the separated limbs of 

 any rigid stratum will tend to approach if compression con- 

 tinues. If the stratum dips steeply the horizontal component 

 of this movement will be small and the vertical component 

 will be masked by erosion ; but if the dip be gentle the rigid 

 stratum, to which no resistance save friction is opposed, may 

 advance over the eroded surface and a thrust-fault be developed 

 on a horizontal or gently dipping plane. Such a thrust-fault 

 might readily escape observation if one end of the rigid stra- 

 tum were pushed upon the other or might be mistaken for an 

 unconformity by deposition if it were exposed at the contact 

 of younger with older strata. 



The second condition, which may lead to the development 

 of a thrust-fault from a stepfold, is fracture. If a rigid 

 stratum be sufficiently bent, in the absence of a load adequate 

 to cause continued flexure, the stratum will break and the 

 crack will offer a plane of weakness on which a thrust-fault 

 may occur. Fig. 3, p. 259. The crack will be radial to 

 the curve and the fault being determined by the resistance 

 offered by the gently dipping limb of the adjacent anticline, 

 will usually have a fault-dip parallel to the dip of that limb. 

 This is the common type of Appalachian thrust-faults. The 

 essential condition, that the rigid stratum should be free from 

 excessive load, may arise either because the deposits over that 

 stratum were never very thick or because erosion cuts away 

 overlying strata on the crown of the arch and thus removes 

 the load. The great Silurian limestone in the Appalachian 

 district of thrust-faults bore from 2500 to 10,000 feet of strata ; 

 and in the district of folding without the development of 

 faults it bore from 10,000 to 30,000 feet of strata. To depths 

 of five miles in the earth the pressure due to gravity is 

 approximately proportioned to the depth and one foot of strata 

 corresponds to about \\ pounds of pressure per square inch. 

 Hence we infer that the great limestone broke and thrust-faults 

 were produced under loads of 2800 to 11,000 pounds per 

 squaie inch, but it folded without breaking under loads of 

 11,000 to 34,000 pounds per square inch. 



The third condition, under which a thrust-fault may develop 

 from a stepfold is that of plastic flow. A sufficiently loaded 

 stratum may be stretched in its overturned limb and so thinned 

 or torn apart as to constitute an overthrust. This type can 

 develop only under excessive load. It is not known in the 

 Appalachians except in the case of coal beds of the Anthracite 



