CHAPTER V. 
GENERAL PETROLOGY. 
The igneous rocks of the Aroostook area are widely separated in 
geographic position, and }^et the distinctive volcanic types are found 
in a rather limited district. 
The granite areas are small and about 40 miles apart. The diabase 
dikes are not numerous, and typically are found cutting the folded and 
slated Aroostook limestone. The lavas — rhyolite, trachyte, and andes- 
ite — are contained within an area about 12 miles by 6 miles, and with 
them are found the most typical tuffs and volcanic conglomerates. As 
the land exists to-day the Castle Hill-Mapleton area appears as a dis- 
tinct center of volcanic activity within which intrusions and extru- 
sions have formed at the same or different times. The presence of 
lavas of different character is not inconsistent with the idea of a lim- 
ited center of activity, and all the field relations indicate a few small 
vents or fissures rather than powerful volcanoes which have ejected 
immense quantities of lava. 
No petrologic work has been done previously anywhere near this 
region, but from the field descriptions of geologists limited areas of 
similar rocks are supposed to exist in northwest New Brunswick, in 
eastern Quebec, and along the lakes in central Maine. Specimens col- 
lected at Temiscouata Lake by Prof . H. S. Williams are andesitic tuffs 
and breccias. Along the coast line the rocks have been studied at St. 
John, New Brunswick, and found to be granite-diorites, gabbros, 
felsite, porphyries, diabases, and porphyrites. ] Farther west along 
the Maine coast aporhyolites, andesites, and other volcanics have been 
described. 2 In the province of Quebec, on the west of Maine, large 
areas of anorthosite exist, some of which have been studied 3 by the 
Canadian surve} r . It is to be hoped that more of these isolated areas 
within the sedimentary slates will be described and souk 1 data secured 
which will permit a discussion of their general petrology in detail. 
The analyses of all the rocks from this region are here given in one 
table, for ready reference. 
i W. D. Matthew, Trans. New York Acad. Sri., Vol. XIII, 1891, p. 185, an<l«V..l. XIV, pp. 187-217. 
2 Bayley, Bull. Geol. Soc. America, Vol. VI, L894, p. 171,. Smith, Geology of the Fox Islands, Maine. 
3 See the writings of F. D. Adams. 
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