408 Eggleston — Eruptive Rocks at Cutting sville, Vt. 



regian Hills lie west of the limit of Appalachian defor- 

 mation, their intrusions penetrating practically undis- 

 turbed beds. The Cuttingsville and other New England 

 examples are located in disturbed rocks, although them- 

 selves not notably affected by deformation. 



Eeference has already been made to the similarity of 

 the Cuttingsville rocks to certain eruptive rocks of south- 

 ern Norway. 



Order of Eruption. — Field observation indicates that 

 eruption of the plutonic types proceeded in the order 

 usual with alkaline intrusions, namely from basic to 

 acidic. Essexite was plainly the earliest intrusion. The 

 syenites followed essexite in a succession not entirely 

 clear. Nordmarkite was apparently the last. Horn- 

 blende-biotite syenite, with pulaskite, nephelite syenite 

 (foyaite) and sodalite-nephelite syenite, probably came 

 between essexite and nordmarkite in about the order 

 given. 



On the other hand, the syenites possibly constitute a 

 single eruption, rather than a succession of intrusions. 

 If the hornblende-biotite syenite is a simple differentiate, 

 rather than a hybrid, it was the first of the syenites to 

 separate. 



The apophysal dikes conform, in a general way, to the 

 same order of basic to acidic intrusions. Essexite por- 

 phyries were the earliest, some of them apparently truly 

 apophysal from the essexite body, and others intrusive 

 into it. Aplite was later than the nordmarkite. The 

 syenite porphyries presumably came between essexite 

 porphyry and aplite. Some of the pulaskite-porphyry 

 dikes, however, cut nordmarkite, and may be even 

 younger than aplite. 



With the "complementary" dikes the order was 

 reversed, the tinguaite dikes, which are relatively acidic, 

 preceding the relatively basic camptonite dikes. Some 

 of the tinguaite dikes are probably younger than all the 

 plutonic types and the truly apophysal dikes. Others are 

 possibly only tinguaitic phases of pulaskite porphyry 

 and nordmarkite. porphyry. The camptonite dikes may 

 not all be of the same age. Some of them are the latest 

 of all the intrusions. 



Origin of the Magmas. — The Cuttingsville eruptives 

 are clearly consanguineous; it may be reasonably 

 inferred that all have come from a single parent magma. 



