332 The American Geologist. December, 1900 
X'inal Haven,* resembles it very closely in mineral composi- 
tion and may well be allied genetically. 
The Monhegan rock is of granitoid structure, and varies in 
color from purplish gray to steel gray, with frequently a green- 
ish tinge, due to the prevalence of secondary green amphibole. 
Weathered surfaces, are grayish brown, and present frequent- 
ly a pitted r.ppearance, owing to the more rapid decomposi- 
tion of the feldspar. 
The mineral constituents most readily discernible with the 
naked eye are feldspar, olivine, pyroxene and hornblende; 
which under the microscope are seen to be in the following 
proportions: plagioclase (50-90 %) > olivine>h)'persthene> 
magnetite > diallage > hornblende. Fluctuations in the 
relative proportions of the ferro-magnesian constituents 
give rise to the subordinate types: troctolyte, noryte, olivine- 
gabbroandgabbro proper. These types represent slight phasal 
differences in the noritic magma, and are so intimately associa- 
ted that no distinctive lines could be drawn between them in 
the field. They pass by gradual transition into hornblende- 
gabbroandgabbro-dioryte,which occupies a more limited area 
at the northern end of the island. Here the rock loses to some 
extent its gabbroitic character, being in part of finer grain and 
variegated color, owdng to irregular inclusions of a quartz- 
bearing, light gray dioryte. 
The petrographic continuity of the Monhegan mass is fre- 
quently interrupted by very coarse grained mineral segrega- 
tions (Schlicren Gauge), and by a series of acid and basic dikes 
which will be discussed in a later chapter. 
Of the minerals composing the olivine-noryte and allied 
rock-types, feldspar is by far the most important. It is of all- 
otriomorphic development, in keeping with the granitic-gran- 
ular structure of the rock. The irregular, stout, tabular cry- 
stals vary in length from two to ten mm and in breadth from 
one to five mm, and show in polarized light well developed 
poly synthetic twinning after the albite and pericline laws. 
Many of these, composite individuals are, furthermore, united 
in accordance with the Karlsbader law. 
*See G. P. Merrill: Proc. of the U. S. Nat. Museum, Vol. VI, No. 
12. 1883; S. E. Dickerman and M. E. Wadsworth: On Olivine Bear- 
ina: Dialaase from St. George. Me., Proc. Bost. Soc. Nat. Hist., Vol. 
XXIII, p. 28; G. O. Smith: The Geology of the Fox Islands, Me. 
Dissert. Inaug. Johns Hopkins University, 1896, pp. 61-63. 
