Gregory.] MAPLETON GRANITE. 147 
the rock are quite marked in their porphyritic tendency. In the hand 
specimen hornblende is seen to be the most common dark mineral, and 
titanite is visible to the unaided eye. None of the minerals, except 
feldspar, arc longer than 3 to 4 millimeters. From this general type the 
rock passes within a few rods into pegmatitic facies on the one hand and 
into a fine-granular variety on the other. In places hornblende is prac- 
tically the only dark mineral, elsewhere the rock appears to be an 
aggregate of mica. Sometimes quartz can not be seen in the hand 
specimen. So decided is the character of these variations that if they 
occurred as independent masses they would be classified as syenite, 
minette, etc. Numerous dark aggregates, such as are described in 
granites of other regions, occur throughout the mass. These, too, 
vary in the proportion and character of their component minerals. 
In thin section, under the microscope, the Mapleton granite shows 
the following mineral composition: Iron ore, apatite, titanite, biotite, 
hornblende, oligoclase, orthoclase, including microcline, and quartz. 
The iron is mostly included in the hornblende. The apatite exhibits 
the usual negative laths. Titanite in well-developed idiomorphic 
crystals is more common than in most granites. Biotite occurs in 
good, clear, strongly pleochroic sections or in stringy and twisted 
strands entwined with chlorite. The hornblende, like the biotite, is 
partl} T developed as irregular masses, sometimes radially grown, and 
partly it is the normal brown-green, pleochroic variety. The oligo- 
clase shows Carlsbad twinning, and measurements according to the 
Michel Levy method indicate a composition of Ab 8 An x . Orthoclase 
shows incipient kaolinization; zonal banding is conspicuous and the 
grains are sometimes broken. Microperthite intergrowths are well 
marked in places, and again are represented by a few albite strands in 
the orthoclase. The microcline exhibits the indistinct, partly devel- 
oped character more fully described as occurring in the granite and 
dikes near Ludlow. Quartz grains are usually the broken pieces of 
larger grains and have the wavy extinction characteristic of quartzes 
subjected to pressure. 
In structure these granites differ from the usual hypidiomorphic 
type. The quartz tends to be idiomorphic against the orthoclase, 
either by developing some crystal edge in the territory occupied by 
the feldspars or by sending branches into the side of the orthoclase crys- 
tals. In places several of the quartz areas extinguish simultaneously 
between crossed nicols. Throughout the whole section the feldspar 
boundaries tend to indenniteness, and there is an approach to the con- 
ditions found in graphic granite. 
The conditions of growth here indicated are different from those in 
ordinary deep-seated granitic rocks, and support the hypotheses founded 
on field observation. The porphyritic tendency is suggestive. The 
larger feldspars seem to have first formed as phenocrysts, since they 
