158 
A GRANITE CONTAINING LITHIA. 
July, 1889. 
Of the accessory minerals the tourmaline has been already 
mentioned. Occasional patches have the dark blue colour of 
indicolite, others are very pale green, almost colourless in 
thin section. 
Topaz occurs sparingly and somewhat irregularly, and the 
grains very rarely show any trace of their crystalline form, 
and even cleavage cracks are not frequent. Here, again, I 
have found only very few and very minute fluid cavities, 
none large enough for any observation as to whether the 
liquid is carbonic acid or not. 
I have also been fortunate enough to get hold of specimens 
of the junction of the granite with the surrounding slate. 
In these, as is usual, the junction is perfectly sharp ; the 
size of the crystalline grains of the intrusive rock is hardly 
perceptibly different up to a small distance varying from ^ to 
J-g of an inch from the slate, then follows a zone of quartz 
and felspar of much smaller dimensions, with a few larger 
crystals, then a very narrow zone of about inch, consisting 
apparently of the same minerals in grains of about the same 
size as those in the previous zone, but the felspars contain 
fewer inclusions, so that the effect is of an almost perfectly clear 
line. Then follows, suddenly, the slate, here changed into a schist 
crowded with tourmaline crystals, the ground being made up 
of a fine mosaic of quartz. It is a curious circumstance that 
specimens of this schist, taken from within an inch of the 
junction, show the presence of litliia when tested in the 
blowpipe or Bunsen burner flame, whereas those at a greater 
distance show no trace of it. This is possibly due to the 
development in the schist of a few flakes of a white mica, 
which, in places, forms veins which look as if they had been 
originally cracks in the rock. The tourmaline shows only a 
moderate dichroism, pale yellow when parallel to the shorter 
diagonal of the polarising nicols prism, brownish yellow when 
at right angles to it. The crystals frequently show traces of 
the terminal planes, and are simply crowded with enclosures 
of some indeterminable mineral ; thev occasionallv have 
spots of a blue colour in the paler mass in the manner so 
common in the mineral. 
A few grains of a highly refracting substance, which 
appears to be isotropic, I should refer to a pale garnet, but 
they show no crystal outlines which would guide one, and are 
quite small. Another specimen of a pale brownish-white 
rock, which occurs near the granite, is a hornstone, but I 
have no knowledge as to the exact relation in space between 
it and the eruptive rock. 
