110 



STRUCTURAL GEOLOGY. 



Figs. 118, 119, 120 illustrate a flexure-fracture and fault along the syn- 

 cline of an overthrust flexure in the Alps, some thousands of feet in span, as 

 figured by Helm. It will be observed that the strata became bent without 



116. 



117. 



Successioi) of monoclines; section across a branching 

 fault. Powell, '75. 



Section across a branching fault. Powell, '75. 



breaking till the flexures of Fig. 118 were produced, illustrating thus the 

 important fact that the bending of flexed rocks has in all cases gone forward 

 with extreme slowness. The plane of the flexure from a to b, between the 



118. 



119. 



120. 



A fold passing into a fault, from the Alps. Heim, '78. 



anticline and syncline, is the plane of greatest weakness, and hence the 

 fracture. There is usually much stretching, also, and thinning, of the beds 

 along the fracture. The fault at the bend in Fig. 120 is an upthrust fault, 

 the stratum m to the right being the same with n to the left ; and the exist- 

 ing distance between the two is a measure of the extent to which the strata 

 were pushed up the sloping fault-plane. Where the flexures are closely 

 crowded together, the faults may divide up a bed into many parts ; and if 

 a bed of iron ore is in the series, its parts may be so far displaced and cut up 

 into so small sections as to make it unprofitable to attempt to follow it. 



The great upthrust faults made along fractures many thousands of feet 

 in depth, like those of the Appalachians, have usually taken place along 

 fracture-planes of small dip — between 20° and 45°. Downthrow or down- 

 thrust faults, however great the displacement, may occur along fracture- 

 planes of all slopes to verticality. 



The region of the great elevations produced along such, faults in the 

 Appalachians has been reduced in general to a level below, that of the 



