210 TRANSACTIONS OF THE [may 21, 
vertical. A large proportion of them measure more than ten 
feet in width, ranging up to forty feet, often with an unknown 
extension under water. Only three or four dikes were found 
that were less than a foot wide, and these seemed to be due to 
local splitting up of larger ones. It is hardly necessary to say 
that allowance must be made for the fact that small dikes would 
be much more easily overlooked than the larger ones. One of 
the latter often makes up more than half of a small, rocky 
island, commonly giving it a decidedly bold and rugged contour, 
resulting from the durable character of the rock and the regular 
jointing. 
As already stated, the rock of the dikes is nearly black, and 
it commonly has a reddish, brownish or purplish tinge. In the 
smaller dikes and on the outer edges of the larger ones it is 
extremely fine grained, but the bulk of the large dikes is made 
up of a rather coarse rock, sometimes showing cleavage faces of 
plagioclase two inches long. 
Under the microscope, sections of the dike-rocks show a holo- 
crystalline aggregate of plagioclase, augite, biotite, sometimes 
olivine, iron ‘oxide and apatite, with a varying amount of alter- 
ation products. The structure is diabase-granular, which, with 
the mineralogical composition, proves the rock to be diabase, of 
which there are two varieties, one with, and the other without 
olivine. Rarely, sections show a marked flow structure and in- 
dications of an approach to the vesicular structure of volcanics, 
while much glass is present. 
The plagioclase occurs in laths showing the common twinning 
lamellae. Extinction angles measured on these average about 
20°, indicating Bytownite. Some of the dikes show large crys- 
tals of plagioclase, tabular parallel to »P%, and cleavage plates 
from these give extinction angles of about 24-26° on »P& and 
12-13° on O P., which would indicate a basic labradorite. 
The feldspar has a brownish, dusty appearance, owing to the 
presence of abundant minute interpositions. With high powers 
some of these are resolved into black dots and blunt rods, 
usually arranged in rows parallel to a pinacoid. Asa rule these 
interpositions are less abundant toward the margins of the 
crystal. They are so small that it is impossible to determine 
their nature with any degree of accuracy, but it is possible that 
the rods are original interpositions, while the finer, dusty ma- 
terial doubtless results from alteration. The feldspar laths are 
often much bent, and nearly always show marked undulatory 
extinction. 
The. outward form of the augite is conditioned by the older 
plagioclase, the former filling the spaces between the crystals of 
