304 GEOLOGY OF OLD HAMPSHIEE COUNTY, MASS. 



PORPITYRITIC CHARACTER OP THE AlVLPHICOLITES. 



The liornbleudic bands of tlie Beniardstou series are often pseudo-por- 

 phyritic, especially west of tlie Mount Hennon School. This structure is 

 widel3^ distributed in older amphibolites from the Hawley to the Conway 

 series, both inclusive, especially in the Guilford and Heath amphibolites 

 and the large upper band of the Hawley schist (see p. 166). It reappears 

 again in the Conway schists in Granville, and is seen in the most striking 

 form in South ^lonson (see p. 249). The dark sui-face of the rock is inter- 

 rupted by white spots 2-10""™ in length, more or less angular and of some- 

 what uniform size. A quite close inspection will often leave the impression 

 that tliey are formed simply by the expulsion of the hornblende needles 

 from the area, and are a portion of the granular base of the rock, but a 

 bright light will show at times the flash of a common cleavage over the 

 whole or lialf of the surface. 



With polarized light the same effect is produced. A simjjle mosaic of 

 feldspar grains appears, but by using a very low power it can generally 

 be seen that the groundmass is held together by a single large feldspar 

 growth, so crowded with foreign bodies that it can hardly be separated. 



I compare these feldspar growths to those described on page 287 in 

 the Bernardston quartzites, or tlie small porphyritic albite crystals in the 

 Hoosac schists, and consider them tlie earlier generation (as compared with 

 the hornblende needles) in their present position. They often include 

 minerals of earl)- growth, as biotite and dolomite (which are now wanting 

 in other parts of the rock), as well as the common groundmass, and have 

 by their early presence prevented the iron-bearing mineral from occupying 

 their place. They are now often saussuritic. made up wholh' or larg-ely of 

 highly refriugent epidote, or zoisite grains, very possibly as the result of a 

 paramorphic change at the time of the development of the hornblende. 



The whole process is one more intelligible as occumng in a calcareous 

 red sandstone than in a metamorphosed diabase, and it is very common in 

 the amphibolites, which occur in thin, extended, conformable sheets, grade 

 more or less into limestone, and show no tendency to form serpentine and 

 steat'te, and it is wanting in the gabbro-like beds and in tlie great Chester 

 amphibolite, which is associated with olivine and enstatite rocks, serpentine, 

 steatite, and emery, and which may thus be derived, at least partly, from 

 beds of distinctly eraptive origin. 



