Alex. Somervail— Dykes in the Lizard Serpentine. 6558 
of interpretation when examined in the usual way with converging 
polarized light, we know that we have before us not an axial section, 
but a mineral of weak double-refraction sliced in another direction. 
In cases where a mineral is of such microscopic size as to be less 
in diameter than the width of one of the chromatic bands exhibited 
by it, we can still easily count the number of chromatic bands that 
come in between the dark line and the thin edge of the wedge, if 
we confine our observations to one colour (red I find the most con- 
venient), and, as we move the wedge forward over the mineral, count 
the number of times it assumes that colour before extinction takes 
place on the micro-crystal entering the dark line or region of no 
double-refraction. 
In the examination of pounded fragments of minerals with the 
aid of a quartz wedge we must carefully consider the element of 
thickness, and we must confine our observations to flat surfaces 
parallel to the plane of the glass slide. 
As an illustration of the close approach to accuracy that may be 
obtained by the use*of the quartz wedge in the way described above, 
I may mention the following example. As Rosenbusch does not give 
the value of y—a for sphene in his Microskopische Physiographie, I 
recently wrote to inquire of a friend whether he could give me the 
required information, and I expressed at the same time my con- 
viction that the double-refraction of sphene was, in intensity, some- 
where between that of zircon and calcite. It turned out that the 
refractive indices of sphene have been worked out by M. Lévy and 
Lacroix, and are given in their book Les Minéraua des Roches just 
published. (y—a) or (m—ny)=0:121. So that sphene really oc- 
cupies the position I had assigned to it on the evidence afforded by 
the rough and ready use of the quartz wedge; the values of y—a 
for the three minerals being: calcite, 0-172 (Rosenbusch) ; sphene, 
0-121 (M. Lévy) ; and zircon, 0-060 (Rosenbusch). 
VI.—On A REMARKABLE DYKE IN THE SERPENTINE OF THE LIZARD. 
By ALEX. SOMERVAIL. 
HE dyke in question occurs at the extreme north end of 
Pentreath Beach, which is about a quarter of a mile to the 
south-east of Kynance Cove. 
The dyke forms a portion of the ‘“granulitic group” of Prof. 
Bonney, which is now known to be of igneous origin, and consists 
of banded gneissic rocks, some of which are like a granite, other 
portions like a diorite, the latter always more or less porphyritic. 
This rather complex rock is here, as in many other localities, thrust 
through the serpentine, but, in this particular instance, it exhibits 
more remarkable features than any other dyke known to me in the 
Lizard area. 
The dyke forms the central part of a small chine or gully. It 
rises from the shore to the top of the cliff, which is about 150 feet 
in height, and can easily be followed throughout its entire upward 
course, as the cliff here is not very precipitous. The decomposition 
