138 The American Naturalist. [February, 
MINERALOGY AND PETROGRAPHY.! 
Petrographical News.— Among the several brochures lately pub- 
lished explanatory of the new map of France, one by Lacroix? con- 
tains two articles. The first is descriptive of the metamorphic and 
eruptive rocks of Ariége, and the second is on the acid inclusions in the 
volcanic rocks of the Auvergne. In the former the marbles of Mercus 
and Arignac are carefully described. In them occur two varieties of- 
humite, brucite, amphibole, phlogopite, scapolite, spinel, corundum, 
sphene, rutile, zircon, and many other less common minerals. One 
variety of the humite occurs in rounded crystals of a clear yellow 
color, that becomie colorless in thin section. The other variety is 
light orange, becoming golden yellow in the section. Both possess the 
same optical properties, except that the orange crystals are pleochroic 
in pale yellow and light golden-yellow tints. They are classed by the, 
author with the clino-humites. Their alteration products are inter- 
esting. The most usual alteration is into brucite, found either in little 
plates, often several millimeters in length, or in fibres forming aureoles 
around unaltered cores of humite. Another alteration is into chryso- 
tile. ‘This is rare, and the change is usually incomplete. A third 
method of decomposition is into a granular mixture of secondary 
calcite, dolomite, and small grains of the original mineral. The 
= amphibole in the rocks is pargasite. Two varieties of spinel were ob- 
served, one a violet and often transparent variety, and the other ge 
pleonast. The violet spinel often accompanies the pargasite and 
. humite. Both spinels are almost always surrounded by a circle of col- 
orless chlorite in thin plates, and this in turn by a zone of secondary =. 
calcite and an outer rim of brucite. The rutile merits special a | 
tion, because what appears to be the ordinary black variety iS found in 
thin section to be sometimes this, and sometimes like the violet rutile 
of the amphibole and pyroxene gneisses of Norway. The ye 
and amphibole gneisses of this region and the wernerite gneisses 
present few peculiarities. The marbles, pyroxene gneisses, and gare 
lites of St. Barthélemy are all marked by interesting features: > 
accessory components of the marbles are almost exclusively graphite 
scapolite, pyroxene, and occasionally oligoclase, the last three forming 
rounded grains rarely surpassing a millimeter and a half in diameter: 
Edited ited by Dr. W, S. Bayley, Colby University, Waterville, Me. 
2 Bull. des Serv. d. 1. Carte. gèol. d. France, No. 11, T. II. 
