feldspar
quarry in Topsham are given on page 124. In the quarry these two rocks
graded gradually into one another, and as shown by the analyses the
proportion of feldspar to quartz is nearly identical in both. The
samples analyzed were obtained by grinding rive or six pounds of each
granite and quartering down the product to a quantity convenient for
analysis. Some allowance must, of course, be made for the difficulty in
procuring a sample that is truly representative of the rock mass. This
difficulty was greater in the case of the coarser rock.
Feldspar "brushes."—A
very uncommon type of graphic granite was observed only in Topsham, in
the G. D. Willes feldspar quarry, where it was exposed on the extreme
southern wall in dikelike bands in the normal pegmatite up to a foot or
so in width. These bands appear to the unaided eye to consist largely
of buff-colored feldspar with different though minor amounts of
biotite. The feldspar forms an aggregate of brush-shaped or long
fan-shaped crystals placed with their long axes at right angles to the
general trend of the dikelike band. A faint banding in these layers
parallel to their general trend and at right angles to the trend of the
feldspar brushes somewhat simulates bedding.
This
banding is the combined effect (1) of a greater abundance of biotite
along certain layers than along others; (2) of the presence of zones
quite even in width, characterized by a coarser intergrowth of feldspar
and quartz than the adjacent layers, though generally showing
crystallographic continuity from one layer to another and even into a
third layer; and (3) of the presence of some parting along planes
parallel to the a pinacoid and resulting slight clouding of the feldspar by alteration along these fractures.
Single
feldspar brushes range in length from a fraction of an inch to 3
inches. The biotite forms thin knife-blade crystals which range in
lenght from microscopic dimensions up to three-fourths of an inch and
are oriented in about the same direction as the feldspar brushes,
penetrating or lying between them.
Under
the microscope the brush-shaped crystals are seen to be made up not of
feldspar alone but of a graphic intergrowth of quartz and feldspar of
microscopic dimensions. The brushlike form represents, however, the
form of the feldspar crystal. Quartz having one optical orientation
frequently extends from one feldspar crystal into a neighboring one.
The microscope shows also that the feldspar is not all of one variety.
That forming the brush-shaped crystals is largely microcline, but some
plagioclase, mostly in aggregates of irregular grains between the
brush-shaped crystals, is associated with it. The plagioclase is albite
and is in places graphically intergrown with quartz. Quartz with the
same optical orientation in many instances continues from a crystal of
microcline into a neighboring one of albite.
