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SUMMARY OF PROGRESS

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Mineralogy and Petrography

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1891.

BY

W. S. BAYLEY.

FROM MONTHLY NOTES IN THE "AMERICAN NATURALIST."

PRICE 50 CENTS.

STANFORD LIBRARY

WATERVILLE, ME.:

GEOLOGICAL DEPARTMENT, COLBY UNIVERSITY,

STAMFORD

From THE AMERICAN NATURALIST, February, 1897

MINERALOGY AND PETROGRAPHY.1

Petrographical News.-Among the several brochures lately published explanatory of the new map of France, one by Lacroix contains two articles. The first is descriptive of the metamorphic and eruptive rocks of Ariège, 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 become 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 interesting. 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 chrysotile. 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 observed, one a violet and often transparent variety, and the other green pleonast. The violet spinel often accompanies the pargasite and humite. Both spinels are almost always surrounded by a circle of co'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 attention, 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 pyroxene and amphibole gneisses of this region and the wernerite gneisses present few peculiarities. The marbles, pyroxene gneisses, and granulites of St. Barthélemy are all marked by interesting features. The 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.

1 Edited by Dr. W. S. Bayley, Colby University, Waterville, Me.

2 Bull. des Serv. d. 1. Carte. gèol. d. France, No. 11, T. II.