294 IRVING. 



phyry is cut by a dike of augite-vogesite, which contains in- 

 clusions of the older rocks. One other instance of the kind is 

 mentioned by Professor F. C. Smith. ^ From these data we 

 can infer that : 



1. The phonolites are younger than the quartz-porphyries and 

 diorite -porphyries, but their relations to the quartz-aegirite -por- 

 phyries and augite-vogesite, is as yet undetermined. 



2. The augite-vogesite is younger than the diorite -porphyries. 

 The vogesite dike is probably one of the latest intrusions, 



and represents the final basic residuum of the magma which 

 has produced the alkaline series. The point which is still 

 left undetermined is the relation between the latter series and 

 the rhyolite-andesite series. Have we here two series of erup- 

 tive rocks marking two widely separated periods of eruptive 

 activity, or a single series which has arisen from the continuous 

 differentiation of a single magma? 



It must be left to further investigation to decide this ques- 

 tion. The entire area of the northern hills, and the associated 

 Warren's Peak uplift must be studied, and an extended set of 

 analyses made before the relations of the rock types can be 

 established. Such an investigation will be extremely interesting, 

 for there is scarcely any one locality which, from its rare types 

 and from its isolated and circumscribed character, will do more 

 to determine the validity of the hypothesis of magmatic differ- 

 entiation than the Black Hills of South Dakota. 



V. ORE BODIES. 



In the district mapped ore bodies occur in all three of the 

 main horizons, Algonkian, Cambro-Silurian and Carboniferous, 

 and in addition there are placers of recent formation. Of these 

 the first was until recently the largest producer, the second is 

 now the most productive, the third is in the early stages of its 

 development, and the fourth is of small importance. 



1 Transactions of the American Institute of Mining Engineers, Vol. XXVII, 

 p. 413, July, 1897- 



