Art. VII.] Watson, Virgilina Copper District. [ 1 1 



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 metalliferous veins of the district. 



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

 compared with analyses of recognized andesites occurring else- 

 where, no marked difterences in the essential constituents are 

 shown. 



Higher SiO, and Al^Oj and lower Fe^O,, FeO, CaO, and 

 MgO in the Virgilina rocks than for the similar rocks in the 

 Catoctin belt are noted. Na^O is approximately the same for 

 the two rocks, with increased KjO shown in the Catoctin an- 

 desite. The Catoctin andesite is characterized by very high 

 iron oxide and correspondingly low SiO^ and clearly represents 

 the basic type of andesite, which readily accounts for the ap- 

 parent variations shown in the comparison with the Virgilina 

 rock. 



Comparing the analyses of the Virginia-North Carolina 

 rocks with those of andesites from Colorado (analysis 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 comoosition which distin- 

 guish a diabase from an andesite. 



So far, then, as chemicil analyses are trustworthy, the 

 percenta<^e ratios ot tlie various constituents in the Virginia- 

 North Carolina rocks, as indicated in I and II, and their aver- 

 age III, are those of andtsitc. 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 AliO,, and MgO. A similar change in the 



