Eggleston — Eruptive Rocks at Cutting sville, Vt. 391 



Sections show a greater proportion of feldspars than 

 simple essexite. About 70 per cent is feldspar, of which 

 approximately one-third is orthoclase and two-thirds 

 plagioclase, of a composition intermediate between basic 

 oligoclase and andesine. There is some microperthite, 

 and possibly altered nephelite and cancrinite. The 

 remaining chief constituents are : hornblende, sometimes 

 surrounded by, and intergrown with, olive-brown biotite ; 

 and augite poikilitically inclosed in hornblende. 



The rock may be regarded as a more acid essexite, 

 which with the hornblende-biotite syenite itself occurs in 

 the series between essexite and pulaskite. 



On the north knob of Granite Hill an extremely coarse- 

 grained phase of essexite is developed. It is largely com- 

 posed of prisms of hornblende, some upwards of 2 inches 

 in length. A little biotite is visible in the hand specimen. 

 The amount of feldspar varies. Towards pulaskite con- 

 tacts the proportion of feldspar increases, and the rock 

 grades into a very coarse hornblende-biotite syenite. 

 This gradation is quite clearly shown near the crest of 

 Granite Hill, from the north knob to the molybdenite 

 prospect (M, fig. 1). 



Porphyritic phases form important parts of the more 

 extensive essexite areas. They are also exposed as 

 smaller masses which appear to be apophyses and 

 detached portions of the main bodies. The groundmass 

 varies from coarse to almost aphanitic. The phenocrysts 

 are usually hornblende, commonly an inch or two in 

 length, frequently much less, and sometimes very much 

 greater. In one case the measured dimensions of a 

 hornblende phenocryst were 15 by 8 inches. Biotite 

 appears, with or without hornblende, usually in the 

 groundmass. 



The porphyritic phases quite generally carry large and 

 small inclusions, apparently of the coarse phases of 

 essexite and hornblende-biotite syenite. These inclu- 

 sions are not always so sharply defined as accompanying 

 inclusions of the country rocks, but they seem to demon- 

 strate an intrusion of porphyritic essexite into an earlier 

 intrusion of more uniformly coarse-grained essexite and 

 possibly syenite. The blurred outlines of the inclusions 

 suggest that the interval between the earlier and the 

 later intrusion was comparatively short. 



