386 Eggleston — Eruptive Rocks at Cutting sville, 17. 



caniptonite may prove to be fine-textured essexite 

 porphyries. 



The dikes range in thickness from a fraction of an inch 

 to 25 feet, with an average of about one foot. The dikes 

 are most numerous in or near eruptive areas. They 

 rarely occur more than half a mile distant. In the erup- 

 tive areas they are not confined to any particular kind 

 of plutonic rock, though they seem to be more numerous 

 near contacts of the syenites with essexite. , 



DATE OF ERUPTIVITY. 



There is little direct evidence bearing upon the exact 

 geological age of the eruptives. They are clearly of 

 later date than the gneisses and limestone of supposed 

 Algonkian age. Their freedom from marked crushing 

 and shearing suggests that the intrusions did not consol- 

 idate until after the strong deformation of the country 

 rocks had been accomplished. Assuming that the last 

 of these could not have been earlier than late Carbon- 

 iferous times, when the whole Appalachian province was 

 greatly disturbed, it may be inferred that the Cuttings- 

 ville eruptives do not antedate the close of the Car- 

 boniferous period. Similar intrusions, at Ascutney 

 Mountain, not more than 25 miles distant, are regarded 

 as probably of post-Carboniferous and pre-Creta- 



PETROGRAPHY. 



Countey Rocks. 



Gneisses. — The gneisses are of two kinds, a basic 

 hornblende-pyroxene gneiss and a mica gneiss which is 

 more or less granitic. The former is the more abundant 

 in the immediate neighborhood of the eruptives, and more- 

 study has been devoted to it on that account. It is 

 typically a heavy, rather dense, fine- to medium-grained, 

 black rock, weathering a streaked or speckled brownish 

 gray. The thin section shows about equal amounts of 

 olive-brown to green hornblende and pale greenish pyrox- 

 ene. In addition there is subordinate brown biotite and 

 plagioclase with some orthoclase and doubtful quartz. 

 Magnetite and apatite are accessory. The rock is 



1 E. A. Daly, U. S. Geol. Surv., Bull. 209, p. 21, 1903. 



