166 W. H. HOBBS REPEATING PATTERNS IN STRUCTURE OF LAND 



folds, seldom occur alone, but rather as elementary parts within series or 

 systems. 



In those districts where, because of exceptionally favorable conditions 

 (such, for example, as the occurrence of several thin and easily recogni- 

 zable but unfolded beds), faults may be identified, small dislocations in 

 great numbers have sometimes been discovered yielding a veritable mosaic 

 of separately moved rock compartments. From the very complexity of 

 these fracture fields, the relatively small individual displacements, but 

 even more the inability to adequately express such structures on the scale 

 of normal geological maps ; such areas have seldom been studied in detail, 

 being generally disposed of by the mere statement that they show local 

 faulting with small displacements and, by inference, are of little signifi- 

 cance in the tectonics of the province. 



THE INHERITED CONCEPTION OF A FAULT 



The common notion of a fault would seem to be that it is a disjunctive 

 plane on which the differential displacement begins at one end, increases 

 to a maximum, and finally disappears or dies out on the continuation of 

 the same line. Once found to have disappeared and its hade and maxi- 

 mum throw measured, its character is supposed to be learned. 



In earlier studies it was natural that many structures not closely related 

 in origin and not properly correlated, should have been hopelessly con- 

 fused. Thus it happens that the word fault is generally applied to any 

 disjunctive plane whatever on which differential movement has occurred. 

 Particularly serious confusion has arisen by the inclusion under this term 

 of the flatly sloping thrust-planes which result from local shearing stress 

 within the under limb of a fold consequent on the folding process. So- 

 called "normal" or "block" faults, on the other hand, do not appear to be 

 connected directly with folding, at least in the same set of beds, but are 

 produced under wholly different conditions of loading. Yet even today 

 "strike" and "cross" faults are terms in common use to describe faults 

 which are either parallel or perpendicular to the strike direction of the 

 folds. Mechanics teaches us, however, that if faults have been produced 

 by the same system of stresses which induced the folds, instead of being 

 parallel and perpendicular to the strike direction, they should cut diago- 

 nally across it. 



JOINTS AND FAULTS COMPRISED IN ONE SYSTEM 



To Daubree, the former distinguished geologist of the Natural History 

 Museum in Paris, we owe a great advance over earlier conceptions of 

 faults, for he recognized clearly that certain systems of joints of which, 



