114 Btdletiji of Laboratories of Deyiison University. [Voi. xn 



rocks of the greenstone area of Michigan has been emphasized 

 by Williams. A second and no less important change in the 

 Virginia-North Carolina rocks is the increased amount of epi- 

 dote 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 KjO is reduced to practic- 

 ally a minimum, while the NaaO is proportionately increased. 

 The constant presence of TiO,^ 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 schis- 

 tose or foliated, and in places thinly fissile, areas of much al- 

 tered, though massive, rocks of the same color and texture are 

 met in a number of places, and are most satisfactorily explained 

 as igneous in origin. This alteration is the result of dynamic 

 metamorphism accompanied by much chemical action, consist- 

 ing largely in the abundant development of chlorite and epidote. 

 A similar change has been observed in the greenstone areas of 

 Michigan^ and South Mountain,^ Pennsylvania. In most cases 

 where the original character is entirely lost and a perfect sec- 

 ondary 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 groundmass of uniformly green and pur- 

 ple colors. The porphyritic structure is more strikingly shown 



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

 ' Bascom, F., Bulletin No. 136, U. S, Geol. Survey, 1896, p. 25. 



