GRAFITE-POEPHYRY. 



223 



Of these branch dikes from the parent stock the longest has heeu 

 traced on the surface for nearly 2 miles, gradually narrowing to a feAA- feet 

 and breaking up into stringers and veins of granite-por])hyry at the north- 

 east end in the same manner as the main dike. Otlicr iiiemlx'rs of this sys- 

 tem of parallel dikes may l)e equally 

 as persistent in llu'ii- trend, without 

 appearing on the surface, erosion hav- 

 ing failed to lay them bare. They vary 

 from 25 to 250 feet in width, but all 

 present much the same physical con- 

 ditions as regards their occurrence and 

 relation to the limestone. On the same 

 general course two naiTOw dikes pene- 

 trate the limestone just west of Castle 

 Mountain, and it is quite possible that 

 they belong to the same system as the 

 others if not actually comiected with 

 them beneath the surface. The absence 

 of offshoots on the west side of the 



main dike and their frequency and per- o'ra/v/e/uorp/ifT^y L'um^tone,^ 



sistency on the east ax'e not without fiq. 5.-G.iiuitc'-porpbyr.v aiko. 



geological interest. By reference to atlas sheet xi the position of the dikes 

 to the main granite-porphyry body and the Fish Creek Moiuitains and 

 Mahogany Hills may be readily seen. 



Limestone adjoining Porphyry.— The main grauite-porphyry dike, like the 

 quartz-porphyry, breaks through the Pogonip, and the smaller dikes on 

 Castle Mountain break through the Lone Mountain, but all other evidence 

 as to their age is wanting. Their mode of occurrence and their relations 

 to the orographic blocks ai*e quite unlike the extnisions of the Tertiary 

 lavas. Although both sides of the main dike lie in limestone of the Pogo- 

 nip epoch, the beds have undergone considerable displacement; those on 

 the east side belonging to a higher horizon than those found in contact with 

 the dike at the north end on the west side. Inmiediately adjoining the 



