Gregory.] DIKES IN THE GRANITE. 149 
zircon. The microcline here and in the quartz-porphyry dikes of this 
district is worthy of special consideration. It is abundant, and is some- 
times partly idiomorphic and distinct in character, but usually is not 
a clear-cut type and is connected with orthoclase as part, or nearly all, 
of a crystal section with no distinct line of demarcation. Sometimes 
it is intergrown with orthoclase and has distinct boundaries between 
the two minerals (see fig. 9). The structure of this granite shows the 
same characteristics as the Mapleton outcrop previously described. 
The term granite is applied to these northern Maine rocks as being 
the nearest approach to the average of the typical exposures. Quartz 
is so often present in small amount and hornblende is so conspicuous 
Lhat they might be more accurately termed hornblende-granite-syenite. 
DIKES OF THE GRANITE AREAS. 
Apllte occurs both on the Aroostook River and at Ludlow without 
characteristics peculiar to either locality. The rock is grayish white, 
Fig. 9.— Microcline and orthoclase intergrowtb. 
mooth, and dense. The only minerals apparent to the unaided eye 
re pyrite and an occasional feldspar which has attained a somewhat 
irger size than its fellows. Closer examination shows that the rare 
ark mineral is usually hornblende, that orthoclase constitutes nearly 
11 the feldspar, and that the remainder is oligoclase in well-developed 
rystal sections with Carlsbad twinning. The quartz takes its usual 
lace as a cement and has a rolling extinction. Little areas of sepa- 
ite quartz grains extinguish simultaneously when revolved with cross 
icols. The rock is seen to consist essentially of quartz and orthoclase 
id tends to a sugar granular structure, and hence is classed as an 
)lite. 
Syenite, as reported by Professor Bailey, 1 is found in a dike in the 
id of the Aroostook at the base of the hill formed of the Mapleton 
-anite. The rock is light gray and fairly fine grained. The dark 
•mponents are collected in small bunches instead of being evenly 
read throughout the more abundant whiter minerals. The rock is 
actically quartz-free and has the characteristics of an ordinary 
enite, being composed of orthoclase, oligoclase, biotite, and horn- 
Bailey, Ann. Repl ,Geol. Nat .Hist. Survey ( !anada, 1887- 88, 1't. M, p. 10. 
