868 STUDIES FOR STUDENTS 



A joint becomes a normal fault so soon as displacement 

 occurs along its walls, and the essential difference, therefore, 

 between normal faults and joints may be said to lie in this con- 

 dition. An area of the earth's crust which has been deformed 

 by jointing under compressive stress is thereby made especially 

 liable to vertical changes of level, because its rigidity, and there- 

 fore its competency to uphold its load, has been reduced. 

 Depression of the area, as a whole (and perhaps elevation of 

 certain portions of it), will take place along the joint planes 

 already formed. There is thus, probably, in all cases at least 

 an infinitesimal displacement along the joint walls, and thus no 

 sharp line can be drawn separating joints from faults. 



Brogger, in his work upon the Langesund-Skien region of 

 southern Norway, says of the joint and fault system: 1 



"The dislocations, that is to say, the faults, have in fact cut the landscape 

 through and through, and not alone parallel to one system of lines, but first 

 chiefly parallel to two principal systems, and then also parallel to other less 

 prevalent directions 



If we consider how thickly the rocks are penetrated by very small faults, 

 it follows, in fact, that a portion of the crust cut up in this fashion is built up 

 like separate rectangular blocks of masonry. 



The study in much detail of the circumscribed area of the 

 Pomperaug valley in Connecticut and its vicinity 2 brought out 

 facts quite independent of, but altogether harmonious with those 

 discovered by Brogger in Norway. 



In the Connecticut region it was shown that a system of 

 nearly vertical faults falls into a number of intersecting series of 

 parallel dislocations, of which four are quite prevalent, and that 

 the directions of the series bear relations to one another, for 

 which an explanation may be found in the mechanics of jointing 

 under compression. A relation was also disclosed between the 

 directions of the faults and the sizes of the included orographic 

 blocks. It was further observed that the system of faults is 

 parallel to and grades into the system of joints, and like it is 



1 W. C. Brogger, "SpaltenveYwerfungen in der Gegend Langesund-Skien," Nyt 

 Magasin for Naturvidenskaberne, Vol. XXVIII (1884), p. 401. 



2 Hobbs, "The Newark System of the Pomperaug Valley, Connecticut." Twenty- 

 first Annual Report U. S. Geo/. Survey, Part III (1901), pp. 1-160. 



