158 77?e American GeolocjUt. March, 1897 
banded veins; showing that this comb-structure and a dis- 
tinct banding are not necessarily correlative, although they 
are undoubtedly of like significance, testifying with equal 
distinctness to successive crystallization. The comb structure 
is also observable in the disposition of other prismatic min- 
erals and of such tabular species as mica and albite, which 
are almost invariably set edgewise or perpendicular to the 
walls in veins of regular form and structure. 
Although occurring but sparingly, vugs or pockets and 
druses are to be regarded as highly characteristic and signifi- 
cant structural features of pegmatite — interesting and en- 
tirely normal possibilities of the pegmatite process. In the 
New Hampshire pegmatites we have found them almost ex- 
clusively in the larger veins and masses; and some of these 
are of large dimensions, as ma}'- be inferred from the fact that 
a pocket in the Palermo mine, in Groton, has afi'orded a well- 
formed crystal of quartz about a yard in diametei'. These 
great pockets are wholly irregular in form, and seem to have 
no definite position in the vein, except that they are never pe- 
ripheral. In other districts, pockets in narrow and distinctly 
linear veins have been observed. These are flatly lenticular 
in form and, except mineralogicallj', in no wise distinguish- 
able from the pockets of ordinary mineral veins. 
Inclosed fragments of the wall-rock are an exceedingly 
common and characteristic feature of pegmatite; and the in- 
clusions are, almost without exception, as so commonly in 
the normal phitonic rocks, in perfect agreement in position 
and orientation of the lamination or other structural features 
with the adjacent wall-rock. This peculiarity of the inclu- 
sions, although not absolutely incompatible with the aqueous 
theory of pegmatite, certainly accords best with the plutonic 
igneous theory. In two instances onlj'- have we observed in- 
clusions that appeared to be foreign to the immediately adja- 
cent part of the wall ; viz., in small veins near the Palermo 
mica mine of North Groton and in the southern part of Graf- 
ton. In both cases the foreign inclusions are of the dark 
hornblendic Bethlehem gneiss, while the wall-rock is fibrolite 
schist in the first instance and the coarse porphyritic granite 
in the second. 
