INTRUSIVE ROCKS. AY 
probably reaches 400 feet or more. Toward the south, in Tourtelotte 
Park, it seems to be somewhat thinner, the average thickness being about 
250 feet. Near the south end of the park the sheet cuts upward across the 
shales and is not found at the lower shale horizon farther south. The 
northerly pitch of the formation, combined with faulting, brings lower beds 
to the surface immediately south of this point, so that neither shale nor 
porphyry is exposed on the east side of the Castle Creek fault, from here 
to the southern end of the district. On the west side of the Castle Creek 
fault, however, the same porphyry appears in Ophir Gulch, and from there 
runs nearly continuously to the southern edge of the area mapped, 
occupying the same geological position as on Aspen Mountain and in 
Tourtelotte Park—near the bottom of the Weber shales. Its thickness 
has been estimated and mapped in this region as varying from about 
300 to 450 feet, but, on account of the numerous parallel faults which 
belong to the Castle Creek system, it is by no means certain that this 
estimated thickness is correct. From here to the south it has not been 
traced, but has been noticed at various points; it seems to thin out and 
disappear, however, somewhere near and to the west of Ashcroft, after 
crossing Castle Creek about 2 miles below that village. 
Northward from Aspen Mountain the extent of the porphyry is con- 
cealed by a fault which traverses the country, with a trend that diverges 
but little from the strike of the beds, and which cuts off more and more of °* 
the sheet toward the north. In this way the porphyry becomes very thin 
im the Smuggler mine, and is last found as a permanent sheet in the southern 
part of the Della 8. mine, where it thins out between the fault below and 
the shales above; northward from this there are bowlders and fragments of 
porphyry in the fault breccia, but the continuous sheet does not reappear. 
What the original extent of the sheet previous to faulting may have been is 
therefore hard to determine. In Hunter Park, however, it has been inferred 
from the minor lithological features of the Weber rocks, and from their 
thickness, that nearly the whole series came to the surface on the west side 
of the fault; and since throughout this district there is no porphyry, it is 
possible that the original sheet came to an end in the neighborhood of 
Smuggler Mountain. 
Description. —The porphyry is found freshest in exposures on the west side 
of the Castle Creek fault, at the southern margin of the Tourtelotte Park 
MON XXXI——4 
