DEFORMA TION OF ROCKS 62 I 



some places in an intermediate area a dozen little overfolds 

 with fault .sli])s may be seen upon a single hand specimen. 

 Hence I conclude that tlie average deformation of a regioji may 

 be tJie same wliethcr it be by a few great faults witli little or 710 

 fissility, by )nore frequent lesser faults witJi or zvithout fissility, 

 by faults and oi' erf olds with or zvithout both cleavage and fissility, 

 or by folding zvitJi or zvithout faidts and cleavage; also that ther-e 

 is every gradation betzveen fiultitig a?id fissility, and probably 

 every gradation betzveen fattlting and cleavage. 



ZONE AFFECTED BY FAULTS. 



A fault may vary in magnitude from a fraction of an inch to 

 many thousands of feet. A fault, like a joint, is limited in hori- 

 zontal as well as vertical extent. It cannot be assumed to extend 

 very far bevond where observed. A fault of an inch may die 

 out within a few inches, both laterally and vertically ; a fault of 

 a hundred feet within a few hundred feet ; a fault of five thousand 

 feet within a few miles. In following a fault longitudinally the 

 throw may be found to become less and less until it is zero, 

 just as a bunch of paper may be torn for a part of its length 

 and the different parts of the torn ends be differently displaced. 

 But while faults may thus die out within short distances, they 

 may have remarkable persistence, both in direction and in 

 length. This does not necessarily imply that they have great 

 persistence in depth, for just as a fold has a tendency to die 

 out, as explained (pp. 210, 211), a fault may also die out below, 

 and sometimes also above. In the latter case the fault usually 

 occurs in a stratum or a formation which is brittle as compared 

 with the overlying rocks. Because of the more plastic charac- 

 ter of the higher strata the deformation there occurs by folding. 

 Probably most faidts at sufficient depth pass into flexures, and 

 deeper dozvn these flexures may die out. As already explained, 

 when a bed is deformed under little weight the strain neces- 

 sarily causes fracture, and the readjustment is largely by fault- 

 ing along the fractures. When all the conditions are the same, 

 except that there is such load that as a whole the rocks are in 



