314 University of California Publications. [Geology 



of the porphyry passes beneath the western edge of the Areturns 

 shaly limestone at a rather low angle. To the west of Copper 

 Flat, however, beyond the northeast-southwest fault shown upon 

 the map, the base of the porphyry mass may be traced without 

 a break for about two miles and a half, resting first upon the 

 White Pine shales, then upon the Weary Flat monzonite. then 

 upon the Ely limestone, then upon the shales again and finally 

 upon the Ely limestone to the north of Rusty Ridge. West of 

 Weary Flat, the trend of this lower edge of the porphyry mass 

 is transverse to both the strike and dip of the strata upon which 

 it rests; and it is evident from the mapping that the rupture, 

 through which the porphyry magma welled up from below, was 

 formed, as in the case of the mass lying in the Ruth syncline 

 quite independently of the preexisting structure as expressed in 

 stratification and flexure. 



On the south side of this western area of porphyry, the upper 

 surface of the intrusive mass cuts across the strata of the Ely 

 limestone in the Rib Hill anticline and appears to pass beneath 

 it. On the western flanks of this anticline it cuts similarly 

 across the strata of the Arcturus formation while plunging be- 

 neath it. At the extreme western limits of the porphyry belt 

 the intrusive mass seems to wedge out rather abruptly but this 

 point did not receive as much attention as the writer would like 

 to have given it. 



From the above summary statement of the structural rela- 

 tions of the porphyry to the rocks which are adjacent to it, 

 there can be left no room for doubt as to its intrusive character. 

 The suggestion that the intrusion is of the nature of a laccolith 

 may, perhaps, be questioned on the ground that the mass cuts 

 across the strata of the encasing formations. That it does so is 

 probably due to the fact that the strata were folded anterior to 

 the intrusion. The laccolithic chamber for the reception of the 

 magma, particularly in the western area, has a floor, which while 

 uneven, appears to have an approximation to the horizontal, and 

 the impression that one gets from the mapping is that the hori- 

 zontal dimensions of the mass are much greater than the vertical. 

 There seems to be little question but that the sedimentary rocks 

 once arched completely over all of the porphyry, although there 



