28o The American Geologist. May, i«9<t 
magnetite. Augite and olivine play a somewhat more import- 
ant role as phenocrysts than does the labradoritc. The olivine 
which in some sections is more abundant than the augite, is 
always idiomorphic and is sometimes completely serpentinized. 
Even in these narrow dikes there is a difference in grain, 
perceptible in the hand specimen, between the selvage and 
centre of the dike (Plate X). In the gneiss in immediate con- 
tact with the dike occur hornblende, biotite and titanite while 
the mass of the rock is plagioclase and quartz. 
On the beach in front of Pemaquid Fort, there is exposed a 
(like of considerable width and hence of coarser grain than the 
dikes of Rutherford's and Thrumbcap islands, but not altogeth- 
er dissimilar in constitution and structure. It has a lighter 
color, due to the predominence of the feldspathic constituent. 
The predominating feldspar, in this rock seems to be andesine 
( Abs Ana). Pericline twinning is a prominent feature and the 
angle of the pericline bands on oio, with the basal cleav- 
age is o°. The maximum extinction angle of the albite bands is 
between 15° and 20". The feldspar occurs as broadly lath- 
shaped crystals. 
The augite is much more altered than in the other dikes. 
The alteration product is chlorite, which has, in some cas^s, 
completely replaced the augite. Olivine is not present. Bio- 
tite occurs as a minor constituent. There is considerable idio- 
morphic magnetite and some apatite. The structure is coarse- 
ly ophitic. The rock, as a whole, is less fresh than is the ma- 
terial of the other dikes. 
It is impossible to make any statement as to the age of this 
dike or of the Rutherford and Thrumbcap diabases. In the 
latter case the microscope shows the material to be as fresh 
and unaltered as is the Newark diabase of the Connecticut val- 
ley. 
