AVARREN.^ — ALKALI-GRANITES AND PORPHYRIES. 237 



iron in excess over ferric as it should be with aegirite absent. The 

 relative proportion of the alkalies is not notably different but their 

 total is higher by 0.65%. Mineralogically these differences corre- 

 spond to much less quartz and to a greater preponderence of albite 

 over microcline than in the coarse-granite, the ratios being — Feldspar 

 to Quartz, 2.S4 (fine-granite) and 1.67 coarse-granite, and Albite to 

 Microcline, 1.25 and 1.02 resp. 



In columns 11 and 12 the peripheral rocks of the Neponset Valley 

 granite intrusion, described by Professor Bascom and already alluded 

 to are given. It can be seen at a glance from these analyses, and the 

 same is fully borne out by the microscopic characters given by Pro- 

 fessor Bascom, that the peripheral phases of the Neponset granite 

 are, like the granites themselves, quite sharply contrasted chemically 

 and Aiineralogically to those of the Quincy, Mass., granite. 



Rosival measurements have also been made on thin-sections of the 

 fine-granite from other localities to determine their approximate 

 mineral composition. These are given below: — 



Mineral Composition of Fine-granites computed from Rosival 



Measurements. 



100.0 100.0 100.0 100.0 



These show a fairly close agreement, but some variation, just as 

 has been shown to exist in the coarse-granite, appears to exist in the 

 fine-granite from different localities. In (a) and (b) the dark silicate 

 is riebeckite ; in (c) and (d) some pyroxene is present, and they are 

 also more altered, while the habits of the hornblende and pyroxenes 

 are not quite so favorable to accurate measurement. 



