792 HENRY S. WASHINGTON 
Both biotites are notable for their poverty in MgO, which will 
be discussed later. They frequently show signs of alteration, 
accompanied by the development of magnetite grains, and their 
laminz are often bent and distorted. 
Secondary biotite occurs in most of the less fresh specimens, 
of a chestnut-brown color and highly pleochroic. It forms 
small flakes arranged perpendicular to or in fan-shaped aggre- 
gates along the edges of hornblende crystals, of which it is an 
alteration product. In the granite from the east side of Wolf 
Hill (Gloucester) are seen small clusters of small biotite flakes 
of an olive-green color and independent of hornblende, which 
are probably primary. 
Pyroxene occurs in a few of the granites, most abundant in 
that of Marblehead Neck, though always in smaller amounts 
than the hornblende or biotite. It is a colorless diopside in 
stout, columnar, rounded crystals, and shows no noteworthy 
peculiarity. Occasionally, as at Trumbull’s quarry, West 
Gloucester, it seems to be derived from hornblende, analogously 
to the often-observed ‘‘magmatic”’ alteration of hornblende in 
eruptive rocks. In some sections of granites which are not 
quite fresh the pyroxene is uralitized to some extent. 
Magnetite is present in most of the granites, but very spo- 
radically and in extremely small amount. Zircon and apatite 
are likewise present in most of the specimens, the former being 
the more common, while apatite is wholly absent in many. 
Allanite occurs in granite from a quarry near the Light- 
house on Eastern Point and in that from Marblehead Neck. 
In the former it is included in hornblende, in the latter in 
biotite. The sections are rather long, with pointed ends, and 
the extinctions, either parallel or at angles up to 36°, show that 
the habit is tabular parallel to @ (100). The color is deep 
brown, with strong pleochroism; a=lght yellowish-brown, 
b =chestnut-brown, C=very dark brown; c>b>a. A zonal 
structure is common, the borders being lighter than the interior. 
Allanite was first observed in Essex county by D. M. Balch’ in 
1Am. J. Sci. (2), Vol. XXIII, p. 348, 1862. 
