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



few thousand of feet — the major faults of the basin — through the faults of 

 only moderate displacement which bound the unit blocks of the western Ore- 

 naug Hill (estimated to be from 10 to 50 feet), and the small displacements 

 which produce hummocks on the southern slope of the eastern twin (figure 38) 

 to, finally, the prismatic joints, which have just been described from the 

 northern end of the same twin." 



EXAMPLE OF A FAULT 8T8TEM 



The basin of Newark rocks in the Pomeraug Yalley has afforded per- 

 haps as satisfactory opportunities for the study of a fault system as is 



Figure 38, — Slope of the eastern Twin of Orenaug Hill 



Larger displacements are represented by scarps hidden beneatli tlie fringes of trees on 

 the borders of the block. Smaller faults have outlined the hummoclis upon the sur- 

 face. (Hobbs, U. S. Geological Survey.) 



anywhere to be found, and though the area comprises only about 20 

 square miles, an entire season was devoted to a careful mapping of the 

 system, probably as elaborate a study of the fault structures exhibited by 

 a small area as has thus far been made. The rocks represented are red- 

 dish conglomerates and shales with black layers of basalt sandwiched in, 

 the one thin and amygdaloidal, the other heavy and massive though with 

 an upper amygdaloidal surface. All formations have been tilted through 

 a small angle (generally 20 to 30 degrees) and all have, further, been 

 protected from extensive erosion through having been dropped on bound- 

 ing faults below the level of the surrounding crystalline rocks. 



The upper and heavier basalt flow stands out in strong relief, so that its 

 fault structure is largely expressed in the details of the topography. The 

 lower basalt flow is hardly 30 feet in thickness and amygdaloidal, but hav- 



