Eggleston — Eruptive Rocks at Cuttingsville, Vt. 409 



No subalkaline rocks have been noted in association with 

 the Cuttingsville eruptives. Yet that fact does not 

 necessarily invalidate the hypothesis that the alkaline 

 rocks are differentiates from modified subalkaline mag- 

 mas. It might be assumed that an originally subalkaline 

 magma was modified by desilication to produce the alka- 

 line parent magma. The desilication may have resulted 

 from the assimilation of limestone which outcrops in at 

 least four different localities about the main eruptive 

 body. More limestone may have been stoped from over- 

 lying beds since removed by erosion. Assimilation of 

 hornblende gneiss, so abundant in the country rock series, 

 may have increased the desilicatiug effect. 



That syntexis of country rock has played a part in 

 magmatic changes is to some extent supported by the 

 frequent development of a quartzose selvage of syenite in 

 contact with quartzose country rock, and by the occur- 

 rence of apparently primary calcite in certain of the 

 dike rocks. 



A highly important factor in the problem of the origin 

 of the magma is the stock structure of the eruptive body. 

 This structure implies a body of magma very much 

 greater than that corresponding to the rocks directly 

 visible at the present erosion surface. Stocks are gen- 

 erally assumed to be small projections from greater 

 batholithic bodies. In any case, the exposed part of a 

 stock is only a transverse section high up in the magma 

 chamber. This condition renders assumptions regarding 

 the nature of the original magma, before assimilation and 

 gravitative differentiation had set in, highly speculative. 



The exposed rocks of the stock have an average com- 

 position not far from that of a typical laurvikose. It is 

 not unlikely that, owing to gravitative differentiation, the 

 deeper rocks are more basic and mafic. This of course 

 would increase the basicity of the stock as a whole and 

 necessitate the assumption of a parent magma more basic 

 than laurvikose. The essexite may represent nearly the 

 chemical nature of the parent magma, suggesting, fur- 

 ther, that the original, unmodified subalkaline magma 

 was truly basaltic. 



This view has much in its favor. It explains the 

 eruption of essexite first as an extravasation of parent 

 magma before very marked differentiation had set in. 

 The succeeding, increasingly feldspathic eruptions mark 



