general
order as in the normal granites, except that the pegmatites are
probably on the average slightly more quartzose, a conclusion that
seems warranted by the numerous transitions from pegmatite masses into
veins composed largely or wholly of quartz. The differences in the
proportions of the principal mineral constituents m the normal
granites and the normal pegmatites seem, however, insufficient to
account for the great differences in their textures. It appears
necessary to seek the cause of these contrasts in differences in the
proportions of minor constituents or in the presence in the granite or
pegmatite magmas of constituents which have since escaped or which,
through occlusion, are not now visible to the unaided eye in the
derived rocks.
MINOR CONSTITUENTS.
The
presence in many pegmatites of unusual minerals, such as fluorite and
other fluorine-bearing minerals, lithium minerals, boron and phosphorus
minerals, and occasionally rare earth minerals, has led certain
geologists ° to attribute to some of these substances an important role
in the production of pegmatite textures. It can not be doubted that
when present in magmas such substances have some influence upon the
texture of the resulting rock. It has not been demonstrated, however,
that the presence of these unusual constituents is essential to the
development of typical pegmatitic textures. In the opinion of the
writer their presence is probably not essential. The pegmatites which
earliest attracted the attention of American mineralogists and
geologists, and which have been most often described in the
literature, were naturally those in which unusual minerals were
present in especial abundance or in perfection of crystal form. Such
pegmatites constitute, however, only an exceedingly small proportion of
the pegmatite in any district and must be regarded as unusual rather
than as normal types. The writer is familiar with certain deposits
showing typical pegmatitic textures, which have been worked for their
feldspar for years with the discovery of few if any of the rarer
minerals.6 In by far the greater number of the pegmatites of
Maine unusual minerals are so uncommon as ordinarily to escape
detection. In pegmatites in which they are present their paucity or
abundance seems to have small influence on the textures developed.
Those inclined to attribute large influence in the development of
pegmatitic textures to the presence of rare constituents usually
contend that a more careful study will show that their scarcity is more
apparent than real. Such an assumption is not in
a
Certain French geologists in particular have supported this view. See
Be Lapparent, Traite" de geologie, 4th ed., 1900, p. 639; De Launay, La
science ge'ologique, 1905, pp. 557-558, 582-583.
b
The Andrews feldspar quarry in Portland, Conn., the Mitchell feldspar
quarry in Maryland, and the Goldings feldspar quarry in Georgetown,
Me., are examples. See Bull. U. S. Geol. Survey No. 420, pp. 50, 75, and this report, p. 105.
