STONE HILL. 201 



grain, but in part they have a more rounded form and show little trace of decompo- 

 sition. In some of those grains there is a central core which is opaque owing to 

 kaolinization (as is the case with the whole grain in the case, of the large fragments) 

 but surrounded by an outer rim of clear fresh feldspar material, which has the same 

 erystallographic orientation as the inner core, the two forming one grain. If these 

 grains are detrital. as they seem to be, there must have been a recrystallization of the 

 old feldspar or a deposition of new feldspar around the old grain. 1 



"In certain finegrained varieties of these Stone hill quartzites the amount of 

 feldspar is very large, and it is difficult to say whether these small grains are in their 

 original detrital shape or are metamorphic. 



"In some cases the large clastic feldspar masses are aggregates of several individ- 

 ual grains of feldspar, forming thus a rock fragment which resembles closely the 

 coarse granitoid gneiss found on Clarksburg mountain to the northeast and Hoosac 

 mountain to the east, which underlies the whole Taconic series. Hence there is a pos- 

 sible derivation for the material of the quartzite. 



"Prisms of tourmaline are common in the rock, and there are occasional rounded 

 grains of zircon. Secondary limouite often stains the rock yellow. Grains of pyrite 

 are abundant in some specimens (locality 18, near top of bill), and in one there is a 

 large amount of calcite present in small grains and irregular masses. 



"These quartzites seem to derive their present materials from two sources, the 

 original detrital material and the material produced from this, at least in part, by 

 mechanical and chemical agencies. The blue quart/, 'pebbles' (locality 628, east side) 

 may be regarded as pebbles whose original outlines have been largely obliterated by 

 mechanical deformation ; the large feldspar fragments are undoubtedly detrital and 

 so is the zircon. The cement or 'groundmass' is composed of detrital quartz and 

 teldspar mixed with an unknown amount of the same minerals formed in situ and by 

 muscovite in large part and tourmaline produced by metamorphism. 



••The distinction made here between clastic and metamorphic feldspar is well 

 marked in the extremes as found on the clastic side in these rocks; on the metamor- 

 phic side in the albite of bhe schists of Greylock and Hoosac mountains, and analo- 

 gous feldspars of the gneisses of Hoosac mountain." 



l Cf. Irving and Van Rise, Bull. U. S. Geol. Survey No. 8, p. H. 



