282 PROCEEDINGS OF THE AMERICAN ACADEMY 



rhombic crystallization. In ordinary transmitted light in thin section, 

 the mineral is olive-green, travei'sed by iron-stained cracks. 



In the pseudo-amygdaloid, while portions of many of the plagio- 

 clase crystals are clear and fresh, they are all more or less changed to 

 prehnite, and exhibit every Stage from the beginning to the perfect 

 pseudomorph. Prehnite also forms countless pseudo-amygdules, from 

 microscopic size up to one-half inch diameter, which have clearly 

 formed at the cost of the plagioclase. Chlorite occurs in the same 

 manner as the prehnite, forming pseudomorphs after plagioclase, and 

 pseudo-amygdules. 



A pseudo-amygdiiloid, south of HougJdon, contains abundant pseudo- 

 amygdules of prehnite, often an inch or more in diameter. Some of 

 these were found to be partially changed to a coarsely foliaceous 

 chlorite. The prehnite has long-radiating tabular structure. The 

 change has begun between the plates, and in places has wliolly altered 

 parts of the prehnite ; the pseudomorphs preserve the same radiating 

 structure as the prehnite. In a moderately thin section, the chlorite is 

 dark green and dichroitic, changing from dark green, when its cleavage 

 lines are parallel to the shorter diagonal of the nicol, to a smoky brown 

 when parallel to the longer diagonal. 



Small scales pressed between slides in balsam revolved dark between 

 crossed nicols; it is therefore uniaxial. 



The feldspar of this rock is apparently either albite or oligoclase, — 

 in all probability the latter; for the highest angle found in ten optical 

 measurements on random sections in the zone I il was 29°. 



Amyodaloids. 



The uppermost zone — the amygdaloid — in many beds is, in several 

 respects, essentially different from the rest of the rock. In these in- 

 stances, the matrix has a much finer texture, often quite aphanitic, even 

 where the lower and pseudo-amygdaloid zones of the same bed are 

 quite coarse grained and distinctly crystalline. The amygdules have 

 generally spherical or ovoidal forms, filling cavities with sharply defined 

 walls. In some rare instances, the amygdules are long and cylindrical, 

 and arranged perpendicular to the plane of bedding. 



In thin sections, the diffei"ence between the texture of the matrix and 

 the texture of the lower zone of the same bed is very apparent. While 

 the primary constituents, when preserved, do not differ apparently in 

 quality, they are of much smaller size, and sometimes show an arrested 

 development and microfluidal structure. Here, too, the amygdules 



