8 BULLETIN OF THE 



bohedral sectious. (See Figure 2.) This structure has been described 

 by many observers and figured by De la Vallee Poussin and Eenard,^ 

 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 Gleitfldche of ilmenite is E, 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 tlie 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 tlian 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 Memoires sur les Caracteres mineralogiques et stratigraphiqnes des Roches 

 dites Plutoniennes de la Belgique et de I'Ardenne franfaise. Me'm. Couronpe's 

 de I'Acad. Roy. de Belgique, XL. 50, 74. 



2 Quart. Journ. Geol. Society, XL. 640. 



8 Cf. Judd, On the Relations between the Solution Planes of Crystals and 

 those of Secondary Twinning. Min. Mag., December, 1886. 

 * Manuel de Mineralogie, p. 430. 

 6 Am. Journ. Sci., (1882,) XXIV. 270. 



