DETAILED DESCRirTION OF SLIDES. 95 



green is less so. On the other hand, the gi-een mineral polarizes in colors 

 of the utmost brilliancy, like those of the preceding slide. 



The hornblendes contain a vast number of included microlites of a 

 black, wholly opaque mineral, crystallizing in needles and long pointed 

 scales, which can scarcely be anything but ilmenite or hematite. These 

 microlites are arranged in certain planes of the hornblende crystals, viz: 

 perpendicular to, and parallel to the base. In sections nearly parallel to the 

 vertical axis no further regularity is perceptible, but cross-sections show 

 that they are also parallel to the prismatic faces and to the clinopinacoid 

 The distances from these faces are wholly irregular, and the effect is there- 

 fore merely that the microlites form with one another angles of nearly 60°. 

 It is noteworthy that just the faces most usually found in microscopic horn- 

 blendes are the ones emphasized by the position of these minute bodies. 

 The same microhtes also occur in the feldspars, in which, too, their distri- 

 bution seems to be governed in part by some crystallographic law, but what 

 one is not evident from this slide. These microlites are, for the most part, 

 entirely unaltered in the brown hornblende, while in the green they are 

 replaced in part by very fine transparent yellowish crystalline grains. In 

 some places the black and the transparent inclusions are continuous with 

 one another, and everywhere the disposition of the latter is precisely that 

 of the former. In fact a narrow inspection does not leave a doubt that the 

 opaque microlites are decomposed into a transparent mineral. The minute 

 size of the grains found does not permit of absolute determination; but the 

 product of decomposition is doubly refracting, possesses a high index of 

 refraction, is slightly dichroitic, and seems to polarize in rather feeble colors. 

 The only familiar minerals which it recalls are titanite and epidote, and the 

 probabilities are that it is sphene. 



In one portion of the slide is a mass of a nearly colorless substance, 

 slightly tinged with green, which seems to be totally isotropic. Under 

 crossed Nicols it remains absolutely dark, and when the quartz plate is 

 introduced, and the Nicols are adjusted to the teinte sensible, no change 

 whatever in the shade is perceptible on revolving the slide This is one of 

 the substances grouped under the tei-m "chloritic constituents," but it does 

 not appear to be certainly identical with the ordinary product of the decom- 



