206 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



very broad band of chiastolite-schists, grading through pimpled schists 

 into the ordinary slate. 



The argillite is itself, in its normal condition, a highly crystalline rock, 

 approaching the mica-schists and widely removed from the more noiTual 

 "argillite," like that of Hoosick Falls, New York. Much of it is pimpled 

 on cleavage surfaces and comparable with the knotenglimmerschiefer of 

 the Germans. 



THE SKRICITE-GNEI8S. 



This rock may be best studied above West Brook village, on the south 

 line of Whately. In the pasture just north of F. Bardwell's the contact of 

 the two rocks can be followed for a long distance, and the argillite extends 

 in a long point sovith into the granite ; and farther south, in the line of con- 

 tinuation of this point, are several masses of the argillite wholly surrounded 

 by tonalite. The southern of -these rises in a vertical wall just east of a 

 small pond in the pasture, and here the exact contact can be studied. The 

 specimens described below were taken from this place. 



The rock at contact is a true sericite-gneiss. The foliation faces have 

 a dull-green, serpentine-like surface, slickensided and with gi-easy feel. 

 Broken transversely the tliick sericite layers fold around small, white feld- 

 spar grains ; other layers run into white quartzite on one side and into a more 

 micaceous and less feldspathic rock on the other. Both varieties resemble 

 exactly the Taunus sericite rocks and are unlike the sericite or hydromica- 

 schists of the west border of the county, where the mica scales are much 

 more distinct. 



Under the microscope the fine-matted felt of a micaceous mineral 

 (sericite) makes a background in which are scattered many wisps of green 

 chlorite ; bright, highly refracting, rounded grains exactly resemble zircon, 

 and large, almost wholly decomposed feldspars. The latter are wholly 

 opaque by transmitted light and rusty white by reflected light, and often 

 show regular eight-sided crystalline cross-sections. When very thin and 

 very highly magnified these sections allow the light to pass through in 

 thin, distant, parallel slits, aiTanged at times at right angles, at times at an 

 angle approaching that of the prismatic cleavage in feldspar. This seems 

 to come from thin bands of the feldspar still undecomposed. The zu'con 

 contains large bubbles. 



