A. Harker—Geology of Mynydd Maur. 225 
hornblende, but in the late Mr. E. B. Tawney’s copy of the memoir 
this is altered in the margin to “ tourmaline.” An examination of 
several thin sections of the rock reveals always both minerals in 
close association and in about equal quantities. 
The microscope shows a fine-grained ground-mass, enclosing, 
besides the hornblende and tourmaline, porphyritic felspars and 
sometimes crystalline grains of quartz. Some of the black crystals 
of the hand-specimens are seen to be hornblende, some tourmaline, 
while others consist of both minerals closely associated, with a very 
irregular or ragged line of junction. The boundaries of the crystals 
show no idiomorphic outline, but an extremely ragged edge, owing 
to their including granules of quartz and felspar similar to those of 
the ground-mass. Most of the crystals, too, both hornblende and 
tourmaline, contain these granules throughout their interior, so 
that the sections, instead of being solid, have a porous or spongy 
appearance. 
The tourmaline is of the blue variety of that mineral, and the 
absorption, for both ordinary and extraordinary rays, is so complete 
that slices of the usual thickness are almost opaque, giving in their 
thinnest portions a deep indigo-blue tint. No traces of cleavage are 
apparent. 
The hornblende has the usual prismatic cleavage well marked. It 
is remarkable chiefly for its abnormal colours and intense dichroism. 
Vibrations parallel to the a-axis of elasticity give a rather light 
brown colour, those parallel to 8 and y a very deep blue, having 
perhaps a greenish tinge in the former case and indigo in the latter, 
but the absorption is so strong that the crystals appear almost 
opaque when the a-axis is approximately perpendicular to the 
shorter diagonal of the Nicols prism. This deep tint is indis- 
tinguishable from that of the tourmaline in the same slides. 
The porphyritic felspars present square sections, twinned apparently 
on the Carlsbad model. They may be orthoclase, but are too much 
destroyed to allow of identification. These crystals are of earlier 
consolidation than the hornblende and tourmaline. 
The ground-mass is a finely granular admixture of quartz and 
orthoclase, as in an ordinary “ micro-granite,” but contaiming in 
addition another mineral which is often plentiful. It occurs in 
minute crystals, of acicular or rectangular form, scattered through 
the ground-mass or included by the other constituents, and having 
usually a fluxional arrangement agreeing with that of the parallel 
hornblende and tourmaline streams. The mineral is colourless, or 
in the larger crystals gives for vibrations parallel to the long axis a 
faint tint of indigo-blue. The erystals give high polarisation colours 
and straight, or nearly straight, extinction. They have a high 
refractive index, which causes them to stand out in relief when 
viewed by ordinary transmitted light. They may perhaps be 
another generation of tourmaline. 
These acicular crystals evidently belong to an early stage of the 
solidification of the magma, and it is important to notice that their 
disposition agrees with the fluxional arrangement of the visible 
DECADE II.—VOL. V.—NO. V. 16 
