io8 Bulletin of Laboratoties of Denison University [Voi. xii 



The feldspars of both the groundmass and phenocrysts are 

 frequently fractured and mashed, showing the effects of pres- 

 sure, which is seen to best advantage in the schistose rocks. 



Not a trace of original hornblende was positively identified 

 in any of the sections. Amphibole is fairly abundant in most 

 of the slides as a secondary product, and as such is usually 

 light green to slight bluish green in color and as fibrous and 

 frayed out masses. A more common occurrence, perhaps, is 

 as a felt of actinolite needles admixed with the other constitu- 

 ents, particularly chlorite, epidote, and iron oxide. The nee- 

 dles are very long and slender and are frequently much curved 

 and bent. The pleochroism of the actinolite is usually quite 

 strong. 



No trace of either augite or olivine was indicated in any of 

 the slides. On account of the greatly altered condition of the 

 rocks, it would not be safe to state that they were not present 

 as original constituents. 



Chlorite is a constant and abundant constituent of the 

 rocks, but is variable in amount, and presents the usual occur- 

 rence for such rocks. A striking feature is the intimately asso- 

 ciated grains and plates of epidote distributed through the chlo- 

 ritic mass in a manner to indicate the simultaneous develop- 

 ment of the two minerals, a characteristic occurrence in some 

 of the Lake Superior greenstones described by Williams.^ 

 Clements^ has shown that in some of the basic vocanics of the 

 Hemlock formation the great abundance of chlorite in some 

 sections is more than could result from the alteration of that 

 amount of the original bisilicate present, and points out that it 

 is derived in part from the altered glassy base. This explana- 

 tion is likely applicable to some of the sections of the Virgilina 

 rocks, since the amount of chlorite is in excess of the original 

 bisilicate, and is probably a derived product in part from an 

 altered glassy base. 



Epidote in the form of small and large irregular grains and 



' Bulletin No. 62, U. S. Geol. Survey, 1890, p. i^6 et seq. 

 * Monograph No. XXXVI, U. S. Geol. Survey, 1899, p. loi. 



