38S Eggleston — Eruptive Rocks at Cutting sville, Vt. 



Quartzite. — Micaceous quartzite forms a few thin beds 

 or lenses in gneiss and limestone. Granular quartzite is 

 relatively abundant as fragments in the breccia, and as 

 xenoliths in the essexite of the northern eruptive area. 



Schists. — Sericitic, chloritic, and tremolitic schists 

 occur only as thin bands between much thicker beds of 

 quartzose gneiss, hornblende gneiss, and crystalline 

 limestone. 



Stock Rocks. 



The rocks of the stocks will be described in the order 

 from oldest to youngest, which is also the order from 

 basic to acid (1, figs. 1 and 2). — Fresh essexite is exposed 

 in cuts on both sides of the crest of the central part of 

 Granite Hill and along the path to the quarry, near the 

 top of the hill. It varies from a nearly black, fairly uni- 

 form, medium- or coarse-grained gabbroid rock, to a 

 gray, more or less porphyritic phase. 



The type phase was taken for study from an open cut 

 or pit (P, fig. 1) on the southwest side of the crest of the 

 middle part of Granite Hill, less than 1,000 feet south of 

 the hornblende-biotite syenite quarry (Q 2, fig. 1). The 

 same phase has been freshly cut on the other side of the 

 crest along the path near the quarry, and rock much like 

 it is exposed by Mill River at the east foot of the south 

 knob. It is dark-colored, sometimes almost black, and 

 usually coarse-grained. Barkevikitic hornblende, show- 

 ing luster-mottling, is generally prominent in the hand 

 specimen. 



Thin sections show coarse-grain and hypidiomorphic- 

 granular texture. About 60 per cent of the rock is feld- 

 spar, mostly plagioclase, ranging from Ab 7 An., to Ab 3 

 An 7 . From 5 to 10 per cent of the rock is orthoclase, 

 which appears in small- and medium-sized grains and 

 irregular areas sometimes intergrown with, enclos- 

 ing, or embaying the plagioclase. The other essentials 

 are hornblende, pyroxene, and biotite, in nearly equal 

 amounts. 



Of the dark-colored minerals barkevikitic hornblende is 

 most notable. Its optical properties are sensibly identi- 

 cal with those of the hornblende in the hornblende-biotite 

 syenite, and will be stated at length in the description of 

 that rock. The pleochroism of the hornblende of the 

 essexite shows, however, more reddish or chestnut tints 



