NA Ne re Fy ee ay Se ee ee E TE 
1886. | Mineralogy and Petrography. 1049 
nodules, but more frequently in seams and veins. In color it 
ranges from sky-blue, through greenish-blue, to dark-green, 
Analyses of specimens of these three varieties yielded : 
Bright blue. Greenish blue. Dark green, 
9.80 19.60 
H,O I : 18.49 
Al,O, $ 36.88 37-88 
Fe,O, 39-53 2.4 4.07 
P,O; 31.96 32.86 28.63 
uO 6.30 7-51 6.56 
SiO, 1.15 -16 4.20 
CaO ati 38 
Upon comparing these results with those obtained by other in- 
vestigators, the authors conclude that normal turquoise can be 
represented by the formula Al, HPO, (OH). The various colors 
which it possesses are probably due to the admixture of a copper 
molecule 2CuO PO; 4H,O. The presence of iron salts would 
tend to give a greenish tinge to the mineral. Under the micro- 
scope it was seen to be composed of minute grains or short thick 
fibers, weakly doubly refracting, with a high refractive index. 
The extinction was parallel to the long axes of the fibers. A 
consideration of the arrangement of the fibers in the veins, the 
composition of the rock in which the mineral is found, and its as- 
sociation with epidote, lead the authors to the supposition that it 
may have been derived from apatite.’ 
PETROGRAPHICAL News.—The gabbros occurring near Balti- 
more, and the hornblende rocks associated with them have been 
made the subject of a bulletin of the U. S. Geological Survey,” 
The treatment of these rocks by the author is very thorough. 
The paper opens with an introduction calling attention to the fact 
that eruptive rocks may, under the influence of heat and press- 
ure, become schistose and in their characteristics very like the 
crystalline schists which have been derived by the alteration of 
aqueous formations. The main portion of the work is devoted 
to the tracing of hypersthene gabbro into a schistose rock, called 
by the author gabbro-diorite. The massive gabbro consist essen- 
tially of a fine to coarse grained mixture of bytownite, light-green 
diallage and hypersthene in varying proportions. In addition 
to these there are also contained in the gabbro a little yellowish- 
brown hornblende, strongly pleochroic and full of minute black 
rock, containing in addition to the plagioclase and hornblende a 
considerable amount of epidote and some garnet, apatite, rutile, 
Sphene, etc. In the case of the alteration of hypersthene the 
author supposes a reaction to have taken place between this 
1 Cf. AMERICAN NATU T, January, 1886, p. 61, 
2 Dr. G. H. Williams, Dollan of the U. S. Geol. Survey, No. 28. 
3 Cf. AMERICAN NATURALIST Notes, March, 1886, p. 275. 
VOL, XX.—NO. XII, 70 
