150 CONTRIBUTIONS TO THE GEOLOGY OF MAINE. [bull. 165. 
blende, with an abundance of pyrite. In composition and structure 
this rock seems intermediate between the aplite mentioned above and 
the kersantite to be described later. 
Grcmite-porphyry. — In the railroad cuts immediately east of Ludlow 
the sedimentary rocks are cut by two classes of dikes. The first class 
includes the aplites, and the second class consists of three narrow 
dikes — 10 feet, 30 feet, and 60 feet wide — of a distinct type of granite- 
porphyry. In the hand specimen the rock appears as a mixture of 
pepper and salt in about equal proportions, in which white feldspar 
crystals have been set. These prominent phenocrysts are not crowded 
together in any part of the dikes, and usually they are separated by as 
much as their own width. No other mineral rises in size above the 
general groundmass. Microscopic examination of a thin section reveals 
the presence of biotite, hornblende, oligoclase, orthoclase, microcline. 
and quartz, with accessory iron, apatite in many minute hairlike crys- 
tals, zircon, and titanite. Biotite is mostly in shreds or laths wit! 
ragged edges, or in clusters and bunches, either of mica alone or 
Fig. 10.— Zonal intergrowth of microcline and orthoclase. 
entwined with hornblende. One agglomerate composed of from twent; 
to thirty pieces of biotite is twisted and knotted together like cords 
Little loose strings of biotite are found which appear to have bee 
torn off from larger pieces. The hornblende is the common gree 
variety, strongly pleochroic, and occurs in short laths or in irregula 
stringy sections. The oligoclase presents nothing of special interest 
Orthoclase, both as phenocrysts and in the groundmass, occurs as well 
formed laths, simple or twinned according to the Carlsbad law. Th 
larger sections especially are altered at the center to kaolin. Whit 
mica is also found along the cleavages as an alteration product 
Microcline occurs in indefinite, partly developed structures in connec 
tion with orthoclase, and also as clear-cut crystal sections, with th 
characteristic cross hatching clearly defined. With the orthoclase 
forms peculiar zonal growths. The outside of a microcline section ma 
be separated from the inside by a zone of orthoclase; or the microclir 
may be completely surrounded by a band of orthoclase. In one cas 
four zones — two orthoclase and two microcline — must be passed ov( 
in going from the periphery to the center of the crystal (see fig. 10 
