396 JS. D. Dana—Taconic Rocks and Stratigraphy. 
slightiy micaceous. The ridge just north of this, situated to 
the west of the village of Great Barrington, consists partly of 
mica schists, of nearly normal type, though over-arenaceous, 
and largely of micaceous bedded quartzyte, with also much 
hard quartzyte especially to the north; and just northwest, a 
low quartzyte area extends to the base of Tom Ball; while 
northeast are others of quartzyte. North of Housatonic village 
quartzyte beds alternate. with true mica schists. Monument 
Mountain has two strata of hard quartzyte with quartzytic mica 
schist between them; but the latter passes into a massive fine- 
grained micaceous rock, gneiss-like in aspect, which is calea- 
reous (effervescing with acid). Hast of Great Barrington, a very 
similar gneissoid rock, somewhat calcareous, is the chief mate- 
rial of the lower part of a high ridge, and shows out gneiss-like 
in the great tumbled masses at the base of the bluff at the north 
end and where quarried; at the summit, the ridge is bedded 
micaceous quartzyte, passing into micaceous or quartzytic mica 
schist. 
The quartzytic mica schists and micaceous quartzyte consti- 
tutes also Rattlesnake Mountain in Stockbridge and other ridges 
farther north to Adams. But for details I refer to the map. 
A microscopic examination of thin slices of the gneissoid 
rock of the ridges east of Great Barrington, and of that in Monua- 
ment Mountain, shows that, besides much quartz, considerable 
mica and a little calcite, it contains also considerable ortho- 
clase, and merits the name of a quartzytic gneiss or a gneissic 
quartzyte. 
In Tyringham there is a well characterized gneiss overlying 
the limestone. 
The destructibility of the bedded quartzyte and the quartz- 
ytic mica schists is generally a marked peculiarity. It is very 
common to find decay extending down one or two hundred 
feet, concealing deeply the solid rock. The decay sometimes 
leaves a hillside covered with loose blocks of hard quartzyte, 
derived from harder layers, or isolated harder portions, in the 
strata. The porosity of the rock renders it permeable by waters 
to great depth ; and then the presence of a little removable cal- 
cite or orthoclase makes destruction easy. 
2. Limits between the limestone and other areas —The approxi- 
mate limits of the limestone areas are in general easily obtain- 
able, as outcrops are numerous. But there are many regions 
where, owing to the covering of drift and the extensive decom- 
position of the rocks (the arenaceous schists and the bedded 
quartzytes), the uncertainties are great. 
The first of these sources of doubt, as I have already said, 
affects widely the west sides of hills and mountains, the side 
that’ faced obliquely the moving glacier. The other may occur 
any where. Rattlesnake Mountain, in Stockbridge, is an ex- 
