126 J-'. BASCOM — VOLCANICS OF NEPONSET VALLEY 



Aggregation a of magnetite, chlorite, nod epidote, as before, represent 

 the ferro magnesian constituent and suggest hiotite by their forms. 



In a cut on the New England railroad near River Street station there 

 £ posed a curiously brecciated porphyry. 



Independently of its brecciated appearance, the rock resembles the 

 purple porphyritic volcanic from Central avenue, Milton. A somewhat 

 lighter shade of purple alone distinguishes it in the hand specimen, and 

 in the slide there is an equally close resemblance. 



There is the same fine grained granular groundmass crowded with 

 lath-shaped and quadratic feldspars, not often showing polysynthetic 

 twinning. 



The extinctions show a soda feldspar, and more nearly correspond to 

 the albite molecule than to any other. 



Former ferro-magnesian constituents are represented by areas of 

 chlorite, epidote, and calcite, with a heavy border of granular magnetite. 



That these albite-orthoclase effusives belong near the trachytes can 

 hardly be doubted. In the absence of a chemical analysis of this ma- 

 terial, no more exact affiliations can be determined. If we adopt the 

 nomenclature used in the case of the aporhyolites and apoandesites, 

 these volcanics will be called apotrachyte porphyry. 



Conclusions 



The volcanics of Neponset valley thus fall into three groups — the 

 apo-soda-rhyolite, apotrachyte porphyry, and apoandesite. They are 

 characterized by a high soda content and by great alteration of original 

 constituents, with the preservation of original structures. 



