154 CONTRIBUTIONS TO THE GEOLOGY OF MAINE. [bull. 165. 
absence of areas about the quartz aggregates which are controlled by 
them in their optical orientation. The group of quartz grains them- 
selves have no collective optical orientation. 
Many of the quartzes are marked by a rude rhombohedral cleavage 
such as has been described by Clements. 1 It is quite possible that the 
rhombohedral cleavage of quartz in some rock sections is formed in 
the preparation of the slide. Feldspars often have their cleavage 
cracks enlarged by grinding, and occasionally a slide shows part or all 
of a crystal torn away. Quartz can be made to cleave by the pressure of 
a sharp point on thin sections, 2 and in the sections of Maine rocks it is 
noticeable that the thicker quartzes have no cleavages developed. 
The groundmass is composed of feldspar and quartz, with a little 
chlorite that is considered as replacing biotite. No other ferromag- 
nesian minerals are present. The feldspars, with plagioclase appar- 
ently predominating, occur almost altogether as short, narrow laths of 
irregular outline that are arranged without order or tend sometimes 
to form radiate groups and flow structures. They never show a ten- 
dency to become phenocrysts. Between crossed nicols the whole 
groundmass is seen to be divided into patches which extinguish as a 
unit and then lighten as the stage revolves. It is evident that this is 
due to a micropoikilitic structure, but one which presents some varia- 
tion from the usual type. The groundmass is of quartz, clear or with 
inclusions, which are arranged in areas with rude mesh structure. 
Within this groundmass the irregular laths of feldspar lie embedded 
without uniform orientation, though often many neighboring individ- 
uals extinguish alike. The quartz areas have no regularity of shape 
or position or size, but extend in all directions, with recesses and 
elongated branches. The different quartz areas are connected by cross 
branches or bridged over by feldspar laths, and as the stage revolves 
one may trace a crooked course following the quartz between the little 
rafts of feldspar across the entire section. This structure is seen in 
ordinary light, but much better between nicols, especially when the 
quartz is so cut as to show its basal section. 
Micropoikilitic groundmasses with similar structure have been 
recently described in rocks of a related character from Michigan and 
California. 3 The Michigamme rock which was first described is a quartz- 
porphyry whose groundmass is almost identical with that of the rock 
from Maine, except that in the latter practically all of the feldspars are 
laths. The California (San Clemente Island) rock is a dacite and 
X J. Morgan Clements, The volcanics of the Michigamme district of Michigan: Jour. Geol., Vol. 
Ill, No. 7, 1895, p. 814. 
2 Mallard, Sur les clivages du quartz: Bull. Soc. min. de France, Vol. XIII, 1890, p. 61. 
3 J. Morgan Clements, op. cit., p. 814. W. S. T. Smith, Geological sketch of San Clemente Island 
(California) : Eighteenth Ann. Rept. U. S. Geol. Survey, Pt. II, 1898, p. 484. See, also, Geology of Santa 
Catalina Island (California) : Proc. California Acad. Nat. Sci., 3d series, Vol. I, No. 1, Feb., 1897, p. 24. 
