CORRELATION OF THE ROCKS. 139 



shales and sandstones, and have 'imparted to them sufficient heat to meta- 

 morphose them for a great distance, indicating the passage of large quanti- 

 ties of molten magma through the fissures, while the lavas of Sepulchre 

 Mountain rest on Cretaceous strata and also cany large blocks of black 

 shale inclosed within them. They plainly show by their crushed and 

 dragged portions that a. profound fault has separated the block of Sepulchre 

 Mountain from that of Electric Peak, dropping the former down consid- 

 erably more than 4,000 feet. Consequently the volcanic rocks of Sepulchre 

 Mountain once occupied a higher elevation than the present summit of 

 Electric Peak and its bodies of intrusive rock. 



In Electric Peak there is a system of fissures that radiate outward 

 toward the south and southwest, as shown by the dikes of porphyry. At 

 the west base of Sepulchre Mountain there is a system of dikes and intruded 

 bodies that radiate outward toward the north and northeast. These fissures 

 antedate the great faulting just mentioned and represent the east and west 

 halves of a system of fissures trending from north and south around to 

 northeast and southwest, which crossed one another at the point where the 

 broadest body of intruded rock is now found. The axis of this system 

 appears to have been inclined toward the east — that is, to have dipped 

 toward the west — and was cut across by the great fault which dropped 

 Sepulchre Mountain. 



The igneous rocks that broke through the strata of Electric Peak 

 consist of a series of andesite-porphyries, occurring in sheets between the 

 strata, and another series of diorites and andesite-porphyries that were 

 erupted through the vertical fissures just alluded to. The central fissure 

 or fissures became the conduit through which the molten magmas followed 

 one another after successive intervals of time. In the outlying narrow 

 fissures the magmas solidified as dikes of porphyry, while within the heated 

 conduit they consolidated into coarse-grained diorites of various kinds. The 

 magmas of this series of eruptions became more and more siliceous. Then- 

 succession is indicated in the table on the next page. 



