8 BULLETIN. OF THE 
bohedral sections. (See Figure 2.) This structure has been described 
by many observers and figured by De la Vallée Poussin and Renard,} 
and by Teall.? The structure has been explained by Teall as due to 
intergrowths of magnetite and ilmenite, according to the fundamental 
rhombohedron. Since the Gleztfldche of ilmenite is R, which is also the 
normal-solution plane,® these may be due to decomposition along the 
normal-solution plane. From No. 222 the heavy portion separated in 
the Thoulet solution was subjected to treatment with the electro-magnet. 
Material was thus obtained so magnetic that, when removed from the 
poles, the grains clung to each other like magnetized iron filings. Treated 
with concentrated hydrochloric acid, this material was strongly attacked, 
but did not entirely dissolve even by continued digestion. 
Professor Wadsworth has described the occurrence of prehnite as a com- 
mon product of the alteration of the feldspar and augite. This mineral 
occurs in veins at the Granite Street quarries, and to determine its char- 
acters a section was made from the mineral obtained from one of these 
veins. The columnar crystals by macroscopic examination seem to have 
their vertical axes, in general, perpendicular to the walls of the fissure. 
In the slide, sections parallel to the long axis (c) always showed a sheaf- 
like grouping of individuals having perfect cleavage, both parallel and per- 
pendicular to the vertical axis. These sections afforded no interference 
figure. Another series of sections (basal) had nearly equal dimensions, 
with two equally perfect cleavages (# P) cutting each other at about 
100°. These sections gave also, in converging polarized light, a very per- 
fect biaxial interference figure, with high positive double refraction and 
orthorhombic dispersion. The optic angle when measured in air was 
found to be 83° 30’, which is much smaller than the results obtained by 
Des Cloiseaux with prehnite from other localities. The plane of the optic 
axes bisects the obtuse angle between the cleavages. The prismatic cleav- 
age is very perfect, hardly less so than the basal. No evidence of twinning 
like that noticed by Des Cloiseaux:* in some specimens, or that found by 
Professor Emerson® in the prehnite of the Deerfield dike, was observed. 
The only section of rock from the region under consideration in which 
1 Mémoires sur les Caractéres minéralogiques et stratigraphiques des Roches 
dites Plutoniennes de la Belgique et de l’Ardenne francaise. Mém. Couronnés 
de l’Acad. Roy. de Belgique, XL. 50, 74. 
2 Quart. Journ. Geol. Society, XL. 640. 
3 Cf. Judd, On the Relations between the Solution Planes of Crystals and 
those of Secondary Twinning. Min. Mag., December, 1886. 
£ Manuel de Minéralogie, p. 430. 
5 Am. Journ. Sci., (1882,) XXIV. 270. 
