366 T. L. WATSON VIRGILINA COPPER DISTRICT 



A cursory examination of the analyses is sufficient to indicate the 

 andesitic character of the rocks, with an advanced stage of alteration 

 shown in IV, V, VI, and VII. Furthermore, the ratio of the Si0 2 to the 

 base-forming elements in I and II, the least altered material, suggests 

 an intermediate rather than an acid or basic andesite. The prevailingly 

 low Si0 2 in the remaining analyses (IV, V, VI, and VII) is explained on 

 the basis of advanced alteration, since the rocks yielding these results 

 were the most altered and were highly schistose in structure. Other 

 apparent irregularities in the analyses are likewise explained on the 

 same basis, since the greatest irregularities are indicated in the analyses 

 of the most altered specimens. To what extent the ore-bearing solutions 

 have aided in the alteration it is not possible to say, but that the change 

 has resulted in part from such action is doubtless shown in the metallif- 

 erous veins of the district. 



When I, II, and III, analyses of the least altered rock, are compared 

 with analyses of recognized andesites occurring elsewhere, no marked 

 differences in the essential constituents are shown. 



Higher Si0 2 and A1 2 3 and lower Fe 2 3 , FeO, CaO, and MgO in the 

 Virgilina rocks than for the similar rocks in the Catoctin belt are noted. 

 Na 2 is approximately the same for the two rocks, with increased K 2 

 shown in the Catoctin andesite. The Catoctin andesite is characterized 

 by very high iron oxide and correspondingly low Si0 2 and clearly repre- 

 sents the basic type of andesite, which readily accounts for the apparent 

 variations shown in the comparison with the Virgilina rock. 



Comparing the analyses of the Virginia-North Carolina rocks with 

 those of andesites from Colorado (analyses IX, XI, and XII) and Maine 

 (analysis X), the differences are by no means so great as shown in the 

 Catoctin andesite, but, on the contrary, the figures are strikingly close 

 and uniform for rocks occurring in areas so widely separated. 



Analyses XIII and XIV are of typical greenstones from the Michigan 

 area derived, as Williams states, from the igneous rock type, diabase. A 

 comparison of these two analyses with the average of I and II given in 

 column III indicates at a glance those differences shown in chemical 

 composition which distinguish a diabase from an andesite. 



So far, then, as chemical analyses are trustworthy, the percentage ratios 

 of the various constituents in the Virginia-North Carolina rocks, as indi- 

 cated in I and II, and their average III, are those of andesite. Passing, 

 then, from the least to the most altered phases of the rocks, the change is 

 observed to consist largely in the increase in the amount of chlorite, as 

 clearly manifested in the assumption of water, hydration ; and also in 

 increased A1 2 3 , FeO, and MgO. A similar change in the rocks of the 

 greenstone area of Michigan has been emphasized by Williams. A second 



