Be ® 
. 348. Gibbs on Tourmalines, &c. 
The red or rose tourmaline of Massachusetts, is found 
chiefly at Chesterfield, in a subordinate bed of granite, con- 
tained in mica slate. ‘The mica slate is the Prgdominant rock 
of the country. It is fine grained, and contains an abundance 
of small garnets. Direction of the strata north and south, 
varying a little easterly; inclination perpendicular. The 
bed of granite is about three hundred feet long, and from five 
to twenty feet broad. It is contained in a narrow ridge of 
“mica slate, which descends into, and is lost in, a valley. The 
sides are precipitous; the highest part is about forty feet 
high. On the east side a considerable part of the granite has 
been destroyed by natural ‘causes, leaving the granite bare. 
The granite consists chiefly of granular feldspar, with grains 
of white quartz, and. a little light coloured mica, is moderately 
fine grained, and of a grayish white colour. In addition to 
tourmaline it contains also emerald, some of the crystals of 
ich are from three to five inches in diameter. 1 suc- 
ceeded in getting one out of its matrix, which is three and a 
half inches in diameter, and its summit (which is a plane 
without any additional facettes) is perfect. 
The tourmalines are contained chiefly in a false vein of 
silicious feldspar and quartz, which begins in the centre of the 
upper edge of the bed of granite, and passes obliquely, de- 
scending to the northeast, about twenty feet, where it is inter- 
cepted from sight by the mica slate. The vein is about one 
and a half foot thick in the upper part, and not more than six 
or eight inches where it is lost. This vein of silicious feld- 
spar contains also a vein of bluish white transparent quartz, 
which is from three to eight inches thick, and passes through 
the centre of the vein of feldspar. 
When | first examined this rock, soon after its discovery 
by Dr. Hunt, of Northampton, I determined the feldspar to 
be a new variety, which has been since confirmed by Pro- 
fessor Hauffman, and now ranks as a new sub-species, under 
the name of silicious feldspar. (P. 41, of the Mineralogical. 
Table.) 
