no. 1835. RELATION OF BORNITE AND CHALCOCITE—LANEY. 



515 



beginning of the volcanic activity there was a period when the vol- 

 canic material was not equal to the land waste, and thus were depos- 

 ited beds of sandstone and conglomerate with only a small amount of 

 volcanic ash. As the activity increased, the amount of land material 

 grew proportionately less and less, until at the time of maximum vul- 

 canism it became nothing, and the normal volcanic beds were formed. 

 As this activity began to diminish, the former conditions began to 

 reassert themselves, and the beds deposited consisted to a greater or 

 less extent of land waste. The following is a somewhat idealized 

 cross-section through the middle of the district at right angles to the 

 schistosity. This section is approximately that exposed along the 

 Southern Railway from a point about ten miles west to about eight 

 miles east of Virgilina. 



The andesitic tuff passes by regular gradation into the sandy and 

 conglomerate rock, so that in the field even where exposures are the 

 best it is not possible to draw a sharp boundary line between the two. 



Sed/menfarj/ beds, /andwaste 



tl/co A/iornb/enc/e Gne/\s>s. : /?S0£ intermixed in i/aryinq pro- 



; port/ons with bc/S/C VO/can/C 

 mater/ a I Highly schistose 

 Qcrorfe porphL/ru. Tuffaceous 



at timers /i/gh/u ^cn/bfose 



Andesite a net andes/~tic 

 tuff Shistose. 



CROSS-SECTION OF VIRGILINA DISTRICT AT RIGHT ANGLES TO THE STRIKE OF THE SCHISTOSITY, 

 APPROXIMATELY ALONG THE SOUTHERN RAILWAY. 



It is possible that the sandy beds may have been formed from the 

 rapid erosion of unconsolidated volcanic ash beds as well as by the 

 commingling of similar material with land waste at the time of erup- 

 tion. Thus in either case it is clear that with an increase of the 

 volcanic material the resulting rock would more nearly approach the 

 true basic tuffs, while with a decrease of this it would approximate 

 more nearly a normal sediment — a conglomerate, sandstone, or shale 

 as the case might be. 



The andesite and the andesitic tuff, especially the former, are the 

 most massive of the older rocks of the region. The andesite is of two 

 types, porphyritic and amygdaloidal, both of winch are mashed and 

 decidedly schistose. The amygdaloidal phase is not abundant, and 

 is usually so highly metamorphosed that it is easily confused with the 

 tuffaceous phase. All the ore deposits thus far developed, and in 

 fact all the prospects as far as known at present, with a very few 

 exceptions, are located in the andesite or the andesitic tuff. 



