206 
Collie—Geology of Conanicut Island , B. I. 
partly due to the inclusions, partly due to the fracturing of the 
mineral All fissures are filled with iron oxide and chlorite, 
thus rendering the mineral dark or semi-transparent. The 
quartz grains are very uniform in size and they rarely show 
porphyritic development. The orthoclase is usually fresh in 
appearance, not having been so greatly altered as the plagio¬ 
clase; it often changes to muscovite, but rarely to chlorite. 
The cleavage parallel to P and M is frequently well shown. 
The lamellar intergrowth with albite is very common, and in 
disturbed areas especially, crystals of microcline are included 
within the orthoclase. Wherever the orthoclase is fractured, as 
it frequently is in the gneissoid granite, calcite fills the fract¬ 
ures. Twinning is comparatively rare and none but Carlsbad 
twins have been observed. In the more disturbed areas there 
are evidences of deformation of the crystals. This is shown by 
the bending of the cleavage lines and by the wavy extinction. 
There are many small cavities lined with flakes of iron oxide, 
whose presence darken the feldspar. This occurrence is proba¬ 
bly due to infiltration of iron into cavities formed by liquid 
inclusions, the Assuring of the mineral having permitted the 
escape of the liquid which originally filled these cavities. As 
a whole the orthoclase retains its original form better than any 
other constituent of the granite. It is less altered by dynamic 
action than the other minerals. The porphyritic crystals of 
orthoclase are not so well developed as the plagioclase pheno- 
crysts, the crystals frequently lack faces and angles, and ap¬ 
pear to be only much enlarged grains, rather than true porphy¬ 
ritic crystals. 
The plagioclase like the orthoclase occurs both in the ground 
mass and in porphyritic crystals. The mineral is abundantly 
developed for a granite. It occurs in angular grains, which are 
on the whole much smaller than those of the quartz and ortho¬ 
clase. 
Albite is the most common mineral of the plagioclase group 
present in the granite, other forms that are found belong to the 
acid end of the series. The plagioclase is usually much altered 
both by deformation and secondary alteration. The porphyritic 
crystals especially exhibit every stage of mechanical deforma- 
