PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 141 



is here aphanitic and crowded with feldspar phenocrysts % of an inch in diameter: 

 large quartz phenocrysts are also conspicuous, especially on the weathered 

 surface, which is light grey, buff, or green in color. The quartz phenocrysts 

 show magmatic corrosion; the feldspar shows secondary alteration to sericite; 

 repeated twinning and an extinction of 14° on 010 indicate an acid plagioclase 

 of about the composition AbgAni. Magnetite, chlorite, and epidote are dis- 

 tributed throughout the finely granular groundmass in large and small aggro- 

 gates. 



Northwest from Bearberry Hill the dike reappears with a granite-porphyry 



fades; the rock closely resembles the normal granite but possesses a conspicu- 

 ously porphyritic texture; phenocrysts of quartz and acid feldspar crowd a 

 microgranitic groundmass of the same constituents; the quartz shows corrosion 

 and granophyric halos; the feldspar is for the most part plagioclase and albitic 

 twinning prevails; albite of the composition AbsAni and more basic species are 

 represented. The ferromagnesian constituent, an inconsiderable component 

 of the rock, is precisely similar to that of the normal granite; in both rocks it is 

 represented by a pleochroic green chlorite associated with magnetite; consider- 

 able secondary epidote is present associated with the feldspar and chlorite. 

 The groundmass shows the granular and the granophyric fabrics. 



In constituents and fabrics the type is more closely related to the granite 

 batholith than to the rhyolitic flows. Farther to the northwest the rhyolite- 

 porphyry facies recurs and is again replaced by the granite-porphyry; thus 

 in a distance of a little more than a mile the dike twice varies from a rhyolite- 

 porphyry to a granite-porphyry. 



Aporhyolite Lava. — The normal granite and the peripheral facies are pene- 

 trated by acid and basic dikes and overlaid by what are now only scattered 

 remnants of once extensive lava flows. The acid lavas, though considerably 

 altered, bear plain textural evidence of their effusive origin; such evidence con- 

 sists in the possession of fluxion, spherulitic, amygdaloidal, and perlitic fabrics. 

 Another kind of evidence is found in the association of pyroclastics with the 

 massive lavas; the coarser pyroclastics are readily recognized in the field by 

 mottled weathered surfaces, produced by the inclusion of variously colored 

 fragments in a light green or pink cement. The massive effusives exhibit a 

 considerable range of colors and of textures ; light green, gray, various shades of 

 pink and purple and a brilliant brick red are the notable colors; an exceedingly 

 dense, cryptocrystalline, felsitic texture, associated with conspicuous fluxion 

 banding, is a widespread type of texture; a feature of the fine-textured lavas, 

 which is also exhibited by modern lavas from the Lipari Islands, is an easy 

 cleavage of the rock into lamellae parallel to the fluxion planes, the rock which 

 is very brittle, also breaks with a conchoidal fracture producing edges so sharp 

 that they cut like glass. 



In some localities, notably High Rock, Hyde Park, the lava is locally an 

 aggregation of spherulites, varying in size from that of a pea to a butternut: the 





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