CHEMICAL CONSTITUTION AND NAME . OP BASIC VOLCANICS 125 



however, have been brought to the rock in solution at the time of the 

 formation of the secondary products — epidote and calcite. 



The analysis falls within the chemical range of the andesites, though 

 the ruling feldspar is more acid than is characteristic of the normal an- 

 desite. While the chemical composition resembles that of the bana- 

 kite dike from Hoodoo mountain,* the Neponset volcanic does not, like 

 the banakite, contain analcite ; it is essentially an altered andesite, rich 

 in soda. In accordance with the proposed use of the prefix apo, these 

 volcanics, which preserve the structures of the original type, while the 

 constituents are more or less completely altered, may be termed apoan- 

 desites. 



Porphyritic Volcanic 



In the woods on the west side of Central avenue, Milton, there is ex- 

 posed a deep purple volcanic with a conspicuously porphyritic structure. 

 The phenocrysts are white, broadly lath-shaped feldspars ; the rock is 

 very feldspathic. It possesses a groundmass composed of a mosaic of 

 feldspar grains, with possibly a little quartz. This groundmass is 

 crowded with feldspar crystals varying greatly in size. Twinning by the 

 albite and carlsbad law are the rule ; pericline twinning is rare. Extinc- 

 tions on 010 and 001 and the maximum equal extinction angle indicate 

 that albite is the predominating feldspar. 



Parallel extinction on some of the microliths indicates that orthoclase 

 is also present. 



Lath-shaped and rudely hexagonal aggregations of magnetite replace 

 some ferro-magnesian constituent whose substance has completely dis- 

 appeared. 



On the east side of the avenue a ledge of breccia furnished a pebble 

 of a similar feldspathic rock. 



It shows some alteration to chlorite and epidote, but on the whole is 

 much fresher than the andesite, and is, like the porphyry just described, 

 very feldspathic. The structure is panidiomorphic and porphyritic. 

 The feldspars, which constitute almost the entire rock, are rectangular 

 and broadly quadratic. They often show a central alteration to epidote 

 or chlorite. 



Repeated twinning is not common and parallel extinction shows that 

 considerable orthoclase is present, though not the ruling feldspar. Max- 

 imum equal extinction varied from 12° to 16° ; extinction on the un- 

 twinned sections was repeatedly 19° or 20°. From this it was inferred 

 that albite was again the predominating feldspar. 



* J. P. Iddings : Absarokite-shoshonite-banakite series. The Journal of Geology, vol. iii, 1895, 

 p. 947. 



