334 ^'''^ American Geologist. December, igeo 
Inclusions of magnetite, pyroxene and feldspar are very 
numerous. 
The olivine decomposes readily to serpentine and brown 
iron ore, and is in many instances surrounded, or entirely re- 
placed, by secondary growths of amphibole, chlorite and 
mica. These secondary products are characteristic of felds- 
pathic gabbros and norytes wherever olivine is in direct con- 
tact with a basic plagioclase, and have been variously inter- 
preted.* 
It seems probable in the case of the Monhegan norytes that 
they are due to the effects of regional metamorphism rather 
than to magmatic corrosion. Where metamorphism is not so 
far advanced as to destroy the original structure of the rock, 
the olivine is seen to be separated from the bytownite by three 
distinct mineral zones. The inner zone adjoining the olivine 
is about i\ mm wide, and consists of tremolite prismatically 
developed normal to the surface of the olivine; the crude 
prisms are perfectly colorless and extinguish at an angle(c:c ) 
of about 1 8°. Following this zone occurs a very narrow one 
(.05-. I mm.) of a colorless, or pale green slightly pleochroitic 
*See A. E. Tornebohm: Uber die wichtigsten Diabas und Gabbro- 
gesteine Schwedens, L. J. 1877, p. 383. 
Fried. Becke. Eruptivgesteine aus der G neissformation des oester- 
teichischen Waldviertels. T. M. P. M., 1882, 'IV. pp. 352-65, and Ges- 
tiene der Columbretes. JNIin. und Pet. Mitth., Vol. 16, 1896, pp. 327- 
336. 
A. Lacroix: Contributions a I'etude des gneiss a pyroxene et des 
roches a werne,rite. Bull. Soc. min. Fr., 1889. p. 245. 
G. H. Williams: The Peridotites of the Cortlandt Series on the 
Hudson River near Peekskill, N. Y. Amer. Jour. Sci.. 1886. Vol. 
XXXI, p. 35. and Bull. U. S. G. S., No. 28, p. 25. 
W. S. Bayley: Fibrous Intergrowths of Augite and Plagioclase in 
a Minnesota Gabbro. Amer. Jour. Sci., Series III, Vol. 43. 1892, pp. 
516-520. 
H. B. Patton: Microscopic Study of some Michigan Rocks. Rep t 
State Board of Geol. Survey for 1891-92, p. 186. 
F. D. Adams: Uber das Norian und Ober-Laurentian von Can- 
ada. L. J. B-B. VIII, 1893. pp. 466-470. 
J. F. Kemp: op. cit., p. 221. 
W. D. Matthews: The Intrusive Rocks of St. John N. B. Trans. 
N. Y. Acad. Sci., Vol. XIII. 1894. PP- 198-201. 
H. Rosenbusch: Micros. Physiog. II. 1896. p. 315. 
R. W. Schaefer: Die basische Gestiene von Ivrea. Min. un.l pet. 
Mitth., Vol. 17, 1898 pp. 495-517- 
J. M. Clements: A Study of Some Examples of Rock \^ariation. 
Jour, of Geol.. Vol. 6. 1898. pp. 384-386. 
A. H. Elftman: The Geology of the Keweenawan Area in North- 
eastern Minnesota (III). Am. Geol., vol. XXII. Sept., 1898, p. 139. 
