214 
to be described by the name of the Tasek-porphyrite. The 
mountain is the relic, left by the erosion, of a thick sheet 
belonging to the volcanic series. The sheet is probably effu- 
sive as the rock contains vesicles, though not in any great 
number. The vesicles are mostly about 1 centimeter long and 
filled with felspar and epidote which have perhaps formed at 
the expense of an original filling of zeolites!. 
The rock contains numerous phenocrysts of felspar imbed- 
ded in a greenish-black, dense ground-mass. These felspar- 
crystals have a thin tabular shape and usually only measure 
1 or 17/2 millimeter in thickness while they may attain a length 
and breadth of 1 or 2 centimeters. Their colour is gray or 
greenish and the bright cleavage-planes parallel to (001) show 
a very distinct twin-striation. The crystals show a strong ten- 
dency to parallel disposition. 
Microscopic characters. — Upon examination under the 
microscope the rock proves so highly affected by contact- 
metamorphism and subsequent alterations that none of the 
original minerals have remained unaltered with the exception 
perhaps of apatite and some grains of iron-ore. The felspar- 
phenocrysts are filled with innumerable minute scales of a 
green mica which also occurs in the ground-mass. Usually 
they also contain more irregularly distributed small grains of a 
mineral which from its refractive power and double refraction 
is supposed to be epidote. Neither the mica nor the epidote 
shows the faintest trace of crystallographic boundaries. The 
clear felspar substance in which these decomposition-products 
are imbedded is twinned according to the albite-law; it has a 
composition between that of an acid andesine and an almost 
pure albite, but on account of the impurity of the crystals an 
exact determination was impossible. The extinction-angles in 
symmetrical sections are small though in some cases they reach 
+ Cfr. A. HARKER, The Tertiary Igneous Rocks of Skye, 1904, р. 50. 
