OF ARTS AND SCIENCES. 273 



with small dull-gray feldspar crystals, and presenting on the fresh 

 fracture numerous irregular-shaped, small, chalky-white spots. Exam- 

 ining the broken surface, under a 1^ inch objective, twin striation is 

 well marked on some of the feldspar crystals, while others are too 

 much altered to cleave, and are easily scratched by the needle. Grains 

 of fresh pyroxene are abundant, and the chalky-white spots are found 

 to be hard, and to resemble in structure, on a minute scale, the common 

 prehnite amygdules of the rocks. There is also an abundance of a 

 velvety, dark-green, very soft substance, with conchoidal fracture and 

 resinous lustre, which is often seamed with a brilliant, black, metallic 

 substance, apparently magnetite. This dark-green substance fills spaces 

 between the other constituents, and often surrounds the prehnite-like 

 bodies. 



This prehnite-like mineral fuses readily in the flame of an alcohol 

 lamp ; it dissolves in heated muriatic acid, leaving pulverulent silica, 

 and, in the filtrate, ammonia produces an abundant precipitate, slightly 

 tinged with brown. There can be little doubt that the mineral is 

 prehnite. 



In thin sections, between crossed nicols, the prehnite bodies give 

 the characteristic colors of this mineral, and have often a radiating 

 structure generally starting from one point on the side. Often they 

 consist of aggregated small masses, with the same structure, and too 

 hard to be scratched by the needle. 



A considerable portion of the feldspar crystals contain a greater or 

 less quantity of these prehnite tufts, and not rarely the only vestige of 

 tiie feldspar left is the contour of its crystal. Optical measurements, 

 by Des Cloiseaux's method, on sections cut parallel to 0, and on random 

 sections in the zone : i.L, together show that the feldspar is labra- 

 dorite. Many of the crystals measured were partly changed to prehnite. 



The prehnite is often changed to a more or less soft, white, semi- 

 opaque substance, and is then almost invariably enveloped by a clear, 

 green soft mineral, with radiating structure, which resembles the 

 chloritic substance of the pseudo-amygdules already described. The 

 plagioclase crystals also contain this green substance, and are, in places, 

 represented by it alone. 



This green mineral gives a greenish-brown streak, and fuses quietly 

 before the blow-pipe to a black magnetic slag; it dissolves in muriatic 

 acid, leaving a deposit of silica, and, in the filtrate, ammonia precip- 

 itates alumina and iron. There can be little doubt tiiat the prehnit- 

 ization preceded and mediated the formation of the pseudomorphs of 

 this green mineral after feldspar. 



VOL. XIII. (n. 8. V.) 18 



