DOME FAULT. 387 



ifoinia gulch, was originally pait of the Iron and Carbonate Hill ridge and 

 that their present separation by the valley of" California gulch is due tc 

 erosion since the Glacial epoch. What is now the main crest of the ridge 

 was once an arm or bay in the Arkansas lake, and the actual rock surface 

 is buried to a great depth beneath the deposits formed in this lake and the 

 later Wash Except, therefore, on the northern edge of the ridge adjoining 

 California gulch, which is the portion shown on the Iron Hill map, data 

 with regard to the actual rock surface are extremely meager. Its geological 

 structure above and to the east is similar to, and practically a continuation 

 of, that of Inin Hill, namely, a series of easterly-dipping beds, capped by 

 porphyry, in which the ore bodies have been developed by following the 

 contact of the Blue Limestone with the overlying porphyry. The main 

 difference lies in the development of the intrusive sheet of Gray Porphyry 

 below the White Porphyry, which is not, however, absolutely parallel 

 with the bedding, inasmuch as on the summit of Dome Hill a small sheet 

 of White Porphyry is left between the Gray Porphyry and the limestone 

 and in the La Plata ground the Gray Porphyr}^ cuts down through the 

 upper part of the Blue Limestone. 



West of the Dome fault the relative position of these two sheets of 

 porphyry affords most valuable evidence as to the underground structure, 

 and actualh' proves a basining-;ip of the beds towards the Dome fault, as 

 has been assumed to be the case in regard to the beds west of the Iron 

 fault. At the Bank of France shaft the Gray Porphyry actually comes to 

 the rock surface. The City Bank and Oro City, on the other hand, pass 

 through the AVhite Porphyry into the Gray, as does the Vining shaft higher 

 up on the hill. The Sullivan, Ben Burb, and Keno shafts have reached 

 the contact and limestone after passing through the White and then a com- 

 paratively thin body of Gray Porphyry. The Blue Limestone is thus 

 shown to be at no great depth below the surface near the Dome fault. On 

 the other hand, at the Coon Valley shaft, near the head of Georgia gulch, 

 the Blue Limestone is over six hundred feet deep, showing a comparatively 

 steep dip from the fault westward.' 



' Stnce the completion of field-work the contact and even valuable bodies of ore have been proved 

 in this region west of the Dome fanlt, notably in the Rosie, Sequin, and Vining claims. In the Sequin 

 the contact was struck at37.T feet, in the Vining at :il7 feet, in each case with a sharp dip.to the west- 

 ward. 



