EMPIRE HILL. 179 



and West Sheridan, and althougli at this point, owing to the fact that gran- 

 ite is on either side and the surfixce is Largely disintegrated, the fault could 

 not be visibly distinguished, it is supposed that the Sheridan fault crosses 

 this saddle to connect with the Weston faidt. "llie White Porphyry dike 

 would thus at first glance seem to be due to an eruption which had taken 

 place along the plane of an already existing- fault ; but the evidence obtained 

 elsewhere all goes to show that the time of eruption of the White Porphyry 

 was entirely antecedent to the action of faulting; and it is therefore more 

 probable that the White Porphyry dike had followed a line of weakness or 

 possible fracture, which in the subsequent dynamic movements would have 

 been more susceptible to faulting than other portions of the formation. 



Between the head of Union gulch and Empire gulch, below the steeper 

 slope of Empire Hill, is a triangular area in which are relics of the lower 

 Paleozoic series, with included porphyries, which have been folded and 

 faulted in an extremely intricate manner. A simple expression of their 

 structure is shown in Section E, in which it is seen that at the foot of the 

 steep slope of Empire Hill a second fault has cut off a portion of a synclinal 

 basin. The upper member in the trough of the syncline is the White Por- 

 phyry, inmiediately overlying- the Blue Limestone. A shaft has penetrated 

 this porphyry into the Blue Limestone below. On the east of the syncline 

 the beds dip 25° to the westward, while on the west side they dip at an 

 average of 10° to the eastward. The southern extremity of the fault is seen 

 near the forks at the head of the north branch of Union gulch, where a little 

 patch of Lower Quartzite rests against the fault, with granite on either side. 



Here occurs a singular eruption, apparentl}^ in the form of an inter- 

 rupted dike, of a rock wliose lithological characters ally it to the Tertiary 

 eruptives. It has been colored on the map as a rhyolite, though it might 

 more strictly be classed as a quartziferous trachyte. It is a rather fine- 

 grained grayish rock, of thoroughly trachytic texture, whose most prominent 

 elements are small glistening hexagonal leaves of biotite ; a few rounded 

 grains of quartz are also visible, and the rest of the rock is made up of small, 

 rather glassy grains of feldspar. Between the crystalline elements is an ill- 

 defined groundmass of gray color. The rock has included fragments of 

 quartzite. Parts of the groundmass are truly microfelsitic, and in some 



