Dikes 7iear Portland, Maine. — Lord. 339 
point on the mainland. The walls of this dike have been ren- 
dered in places quite schistose by extensive lateral pressure. 
The rock is very coarse grained and shows macroscopically a 
"beautiful graphic intergrowth of quartz and feldspar. On 
microscopical examination this feldspar is seen to be 
microcline, penetrated by irregular lamellae of albite. Be- 
sides quartz and this microcline (perthite) the rock contains 
some muscovite, and a few grains of accessory apatite. 
The aplyte is a white, even granular rock with peculiar 
^accaroidal texture, and consists mineralogically of micro- 
cline, orthoclase, albite, nuiscovite, garnet and apatite. The 
garnet is in the form of very regular octahedra of a light pink- 
ish color. 
Examples of olivine-diabase-porphyry are from dike i, 2, 
3, 4, 5. 6, 7, 10, II, 13, 14, 15, 16. 17. 18, 19, 20, 21, 22, 23, 24, 
26, 29, 30. 
The specimens vary in color from steel gray to bluish 
black. On exposed surfaces they weather brown. ■ The habit 
of the rock is in general basaltic with phenocrysts of olivine. 
feldspar, augite, magnetite and irregularly shapped masses of 
pyrite. With the aid of the microscope, the ground mass re- 
solves into a panidiomorphic aggregate of augite. lath-shaped 
plagioclase, magnetite and apatite. It is in every instance 
holocrystalline, although frequently the rock itself appears on 
exposed surfaces to be scoriaceous, owing to cavities having 
been formed by the decomposition and leaching out of the oli- 
vine phenocrysts. 
The olivine is exclusively of intratelluric origin, and, when 
well preserved, the characteristic planes 100, no, 021 could be 
easily identified. (7, 10, 21, 22, 24, 26.) Frequently, how- 
ever, products of decomposition obliterate the crystal form, 
leaving in place of the crystal a fibrous aggregate of sepen- 
tine, accompanied in many cases by calcite and epidote. The 
amount of olivine varies considerably in the different speci- 
mens. 
The atigite is the most important mineral constituent of 
these rocks. The crystals are either prismatic in form with 
octagonal cross-sections, owing to equal development of no. 
with 010 and 100, or they are somewhat tabular formed by the 
pinacoid 010. Twinning after this face is not uncoinmon. 
