Origin of Pecpnalite. — Crosby and Fuller. 175 
of feldspar (red orthoelase) on either wall, with (juartz and 
sometimes epidote and allanite in the middle. Long, narrow 
pockets are a very characteristic feature of these banded vein- 
lets of pegmatite, indicating a high degree of hydration of the 
magma. 
Although the fluidity of the highly hydrated pegnjatite 
magma would make extravasation easy, and thus, perhaps, 
readily account for the size and number of the pegmatite 
veins in the surrounding formations, and the fact that they 
sometimes extend to great distances from the parent plutonic 
mass; yet extravasation of a definite pegmatite magma is out 
of the question in the Marblehead and Swampscott occur- 
rences, and the pegmatite must be due essentially to a dilfer- 
entiation in sifa of the normal granite magma, the only cause 
of the differentiation which we are able to recognize being 
the absorption of water from the wall-rock. While studying 
the pegmatites of New Hampshire we were strongly impressed 
as already noted, by the number, size and general prominence 
of the pegnatitic veins in the schists, even at considerable 
distances from the mother rock — the Concord granite, the 
pegmatite being quite in contrast, in this respect, with the 
normal granite, which seems to form but few apophyses or 
dikes in the schists. In fact, we believe it may be truly said 
tliat while pegmatite intrusions are especially characteristic 
of the schists, intrusions of the normal Concord granite occur 
chiefly in the more massive formations ( Winnepesaukee 
gneiss, porphyrytic granite, etc.). These considerations have 
recently suggested to us the following modification or exten- 
sion of the pegmatite theory : 
Pegmatite magma maybe formed in two distinct ways: 
First, by normal magmatic differentiation in a boss or large 
body of magma, both crystallization and the operation of 
Soret's principle tending, as already explained, to increase the 
degree of hydration of the magma residuum about favorable 
centers. These magma residues may crystallize in situ in the 
midst of the previously solidified normal granite, giving rise 
to what may be called sedentary pegmatite, which is especial- 
ly characterized by its gradual passage outward into the 
normal granite; or the pegmatite magma maj' suffer extrava- 
sation, finally crystallizing in spaces of discission or dissolu- 
