82 
Journal of the 
patches. On the fresh surfaces of fracture there may also be 
distinguished small scattered irregular plates of black mica. The 
dyke is jointed in the direction of strike. 
Microscopic Examination .—I prepared twelve thin slices from 
samples collected at various places along the course of this dyke. 
The ground mass is microcrystalline, and its composition is as 
follows:— 
(a). A small amount of yellowish glassy basis which is, of 
course, isotropic. 
( l>). Very numerous minute prisms of colourless felspar, some 
of which are twinned. 
The angle formed in these prisms by the plane of vibration, as 
against the composition face, I found to be small, but I was not 
able, owing to the minute size of the crystals, to obtain any 
reliable measurements. I regard these felspars as triclinic, and 
the small angles point to one of the soda-lime varieties. Very 
frequently these felspars form radiating groups. 
(c). Very numerous minute prismatic crystals having inclined 
terminations, they vary individually from mere fibres up to 
dimensions of -005 by -0005 inch. They are green in colour, and 
are dicliroic. I have observed cross sections of the larger indivi¬ 
duals which showed me the angle of hornblende. Such an 
instance I give in Fig. 1. I could not observe the characteristic 
prismatic cleavage in these microliths. Some of the longitudinal 
sections show divisions at a somewhat acute angle to the axis c, 
similar to such divisions as seen in actinolite. These microliths 
are, I doubt not, some form of amphibole. 
They lie in the ground mass at all angles to each other, and 
often form a perfect network. In slices taken from the northern 
end of the dyke, these microliths have a somewhat different 
appearance. Besides the long prismatic forms having inclined 
terminal planes, there are also groups of irregular plates; both of 
these forms are brownish in colour and dicliroic. The plates 
somewhat resemble mica, but there are no cross sections which 
support such a view, and the dichroism of the plates is incom¬ 
patible with their being mica, as they would then be seen in the 
direction of the axis c. 
These microliths clearly show the flow structure of the ground- 
mass, as they are in places arranged in streams having their longer 
