Microscopical Society of Victoria. 
83 
diameters parallel. These streams divide at some of the larger 
minerals, and again reunite beyond them. 
(a). Small rhombic plates of brown magnesia mica, which is 
strongly dichroic. 
There are traces of alteration and decomposition in some parts 
of the ground-mass. It becomes diin and turbid, owing to the 
presence of a grey, cloudy substance, which is also specially seen 
in the felspars. This is probably kaolin. In other places, the 
base is made cloudy by a light brown pigment, which follows the 
minute cracks in the contacts of the felspar crystals with the 
ground-mass, and is also connected with groups of the ampliibole 
microliths. This is, no doubt, ferric hydrate. 
In this ground-mass are porphyritic crystals of: 
(rt). Felspar . — Porphyritic crystals of felspar are predominant in 
most parts of the dyke. Their sections, seen in the thin slices 
which I have prepared, differ in character. In one kind, the outlines 
indicative of the crystalline form are sharply defined against the 
ground-mass, as is shown in Fig. 3, and some of these sections have 
cleavage planes perpendicular to each other. Occasionally, there 
are irregular groups of simple crystals. Whenever sections occur 
which can be referred to the zone o—zV, the plane of vibration is 
found to be parallel to the edge o — i. It is of the utmost rarity 
that any intergrowth with albite can be suspected from the occur¬ 
rence of minute lamillm, interpolated parallel to the place ii. 
The felspar substance is clear, translucent, and colourless, and 
much resembles in its translucency, its homogeneous character and 
its mode of twinning the potass felspar adularia. A. qualitative 
examination of one of these porphyritic crystals confirmed the 
belief that it is a potass felspar. 
It is not unfrequenfcly the case that these orthoclase crystals are 
twinned, according to the Carlsbad law, and I have observed one 
instance of a Baveno twin. 
Besides these larger orthoclase felspars, there are more rarely 
other smaller crystals, which are certainly triclinic. They are 
usually of less well-defined outlines, and are more affected by 
alteration than the former. They are frequently bordered by a 
dark-coloured (blackish-green) choritic substance, which also 
extends into the cleavage planes and fissures traversing the 
crystal. Portions of the ground-mass, together with ampliibole 
