DISCUSSION OF CHEMICAL ANALYSES 367 



and no less important change in the Virginia-North Carolina rocks is the 

 increased amount of epidote in the much-altered phases of the rocks, a fact 

 indicated microscopically as well as in the field, and further confirmed 

 chemically in the greatly increased amounts of CaO in the analyses of 

 the altered over those of the fresher rocks. 



Attention is finally directed to the alkalies' ratio in these rocks, in 

 which it is observed that K 2 is reduced to practically a minimum, while 

 the Na 2 is proportionately increased. The constant presence of Ti0 2 

 and MnO in the analyses is a noteworthy feature. 



Evidences of Eruptive Character 

 field evidence 



The field evidence that the schistose rocks here studied are of igneous 

 origin is not entirely lacking when the belt as a whole is considered. 

 While the rocks are prevailingly schistose or foliated, and in places 

 thinly fissile, areas of much altered, though massive, rocks of the same 

 color and texture are met in a number of places, and are most sat- 

 isfactorily explained as igneous in origin. This alteration is the result 

 of dynamic metamorphism accompanied by much chemical action, con- 

 sisting largely in the abundant development of chlorite and epidote. A 

 similar change has been observed in the greenstone areas of Michigan* 

 and South mountain,f Pennsylvania. In most cases where the original 

 character is entirely lost and a perfect secondary schistosity assumed 

 it becomes necessary to resort to the microscope to determine their 

 nature. 



In the massive and least altered phases of the rock the porphyritic 

 structure is apparent. The porphyritic constituent measures less than 

 one millimeter in size, and is distributed through an aphanitic ground- 

 mass of uniformly green and purple colors. The porphyritic structure 

 is more strikingly shown in some of the thin sections under the micro- 

 scope than in the hand specimens. In such cases the porphyritic min- 

 eral consists principally of a well striated plagioclase. 



The prevailing fineness of grain of these rocks, which is equally char- 

 acteristic of the freshest specimens as for the most altered material, and 

 the associated tuffs or clastic volcanics suggests solidification at the 

 surface. 



The weathered outcrops afford, as a rule, only slight indication of an 

 igneous mass, although at one point a few miles to the south of Vir- 

 gilina, in Carolina, the spheroidal type of weathering was observed. 



* Williams, G. H., Bulletin no. 62, U. S. Geol. Survey, 1890, pp. 192-217, 

 fBaseom, F., Bulletin no. 130, U. S. Geol. Survey, 189fi, p. 25. 



