338 The American Geologist. December, \m\ 
of prismatic habit and dark green color. Pleochroism is 
strong: parallel c and lj=dark brownish green, par- 
allel a=greenish yellow. Absorption (c>b>a), and 
an extinction angle (c:c) of about i8° are characteristic of dior- 
itic hornblende. 
With the increase of this primary amphibole the gabbroitic 
character of the rock changes until finally a rock is reached 
composed entirely of this mineral, associated with plagioclase, 
reddish brown mica, magnetite, apatite and some little quartz. 
This is the most acid rock of the area studied, and differs from 
a typical dioryte solely in the basic character of the plagio- 
clase. 
A specific gravity determination of this feldspar gave 
about the same result as that obtained for the bytownite (sp, 
gr. 2.681). With the aid of the microscope, however, it was 
found to consist of a larger interior portion, with the optical 
properties of bytownite and a narrow outer shell, perfectly 
free from inclusions, and having the small extinction angles 
of andesine (a: a =^about 4° on (001) and 9° on (010). 
A striking feature in the geology of Monhegan are the 
dike-like masses (segregation veins)already referred to (p. 332) 
Intrusions of this kind are very common in gabbro areas gen- 
erally and have been treated at length by various authors.* 
On Monhegan two distinct types were readily rec- 
ognized; one composed almost entirely of bytownite (labra- 
dorfelse) — being by far the most common in the region, and 
*See G. H. Williams: Bull. 28, U. S. G. S., p. 25. 
F. D. Adams: loc. cit., p. 449. 
W. S. Bailey: The Basic Massive Rocks of the Lake Superior re- 
gion. Jour, of Geo!., Vol. Ill, pp. 815-24. 
A. C. Lawson: Rept. on the Geology of the Rainy Lake Region, 
Geol. Survey of Canarla, Ann. Rept.. 1887, Vol. IIL Appendix L, p. 73. 
A. I H. Elftman: Notes upon the Bedded and Banded Structures of 
the Gabbro, etc. Geol. & Nat. Hist. Survey of Minn., 23rd Rept., 1894, 
pp. 224-30, and Amer. Geol., 1898. Vol. XXII, pp. 135-43. 
Alfred Harker: Carrock Fell: A study in the Variation of Igneous 
Rockmasses. Quart. Jour. Geol. Soc, London, "1894, p. 316. 
J. W. Judd: op. cit., p. 49. 
A Geikie and J. J. Teall: On the Banded' structures of some Ter- 
tiary Gabbros in the Isle of Skye. Q. J. G. S., 1894, pp. 652-53. 
C. Chelius: Lucitporphyrit, ein Ganggestein von Ernsthofen und 
seine Beziehungen zu den aiideren Diorit und Gabbro Ganggesteinen 
des Odenwaldes. Notizhl. d. ver. f. Erdk. d Grossh. geol. Landesanst. 
zu Darmstadt, IV. Folge 18. pp. 14-22. 
C. F. Kolderup: Die Labradorfelse des westlichen Norwegens. 
Neues Jahrb., 1899. Vol. i, p. 451. 
