374 



FREDERIC H. LAHEE 



As for the groundmass, it consists of abundant quartz and 

 chlorite, with some sericite and feldspar. It is so fine that micro- 

 scopic discrimination between quartz and feldspar is almost 

 impossible. Both chlorite and sericite show some tendency to 

 parallel arrangement. The chlorite is likely to occur in larger flakes 



in the lee of the phenocrysts and in 

 the quartz embayments — in other 

 words, where there was some protec- 

 tion from the shearing stress, such 

 as it was. Neither quartz nor feld- 

 spar phenocrysts contain scattered 

 inclusions of the groundmass. 



The porphyry from the channel 

 of the brook running into Young's 

 Pond is somewhat more sheared than 

 that from the schoolhouse ledge. The 

 quartz and feldspar phenocrysts are 

 more strained and granulated, and in 

 some places the broken fragments 

 are separated by a strip having cata- 

 Sericite is more 



Fig. o. — A plagioclase pheno- 

 cryst showing the bundles of seri- 

 cite laths that wrap round it at 

 the poles oi maximum compression. 

 Enlarged n diameters. 



clastic structure (Fig. n) 

 abundant and has better orientation. It is 

 often thickly plastered against the pheno- 

 crysts about which it wraps at the poles of 

 maximum compression (Fig. 9). In other 

 respects this schist is similar to the less meta- 

 morphosed specimens. 



Pebbles of the conglomerate seliisl of the 

 Young's Pond locality. — Pebbles of the por- 

 phyry schist are identical with the bedrock 

 of the same. They need no further descrip- 

 tion here. 



The dark masses which have been mentioned as greenish when 

 fresh, but drab when weathered, are of peculiar interest, since 

 they have been regarded as highly metamorphosed blocks of argil- 

 lite. They have been compared with the slate blocks in the Squan- 

 tum tillite. 1 Thin sections were prepared from several fragments 



1 R. W. Sayles, op. cit. 



Fig. 10. — Plagioclase 

 phenocryst with zonal 

 structure and crystal 

 form. The inner, stip- 

 pled part of the crystal 

 is somewhat decayed. 

 Enlarged 32 diameters. 



