14 PROF. T. G. BONNEY ON THE GEOLOGY 
the tints of moderate brilliancy, varying from pale green to pink, 
and extinguishing when the cleavage-planes are parallel with the 
vibration-planes of the crossed nicols; probably a hydrous potash 
or soda-mica; (6) a dark brown mica, associated with, and often 
rendered almost opaque by, the black dust named above, probably 
akin to biotite, or a hydrous variety of it; and (c) a pale olive-green 
slightly dichroic mineral in rather smaller flakes which seems not 
to extinguish in quite the same positions as the first mineral, and 
so may be rather a chlorite than a mica. 
The white and the dark mica are closely interbanded, sometimes 
the former, sometimes the latter predominating, in which case the 
slides become locally only very feebly translucent. The crump- 
ling of these interlaminated minerals is often singularly beautiful, 
equalling the most complicated arrangements of banded agates (see 
fig. 7). In this case [have observed that the white mica, when seen 
with the polarizing apparatus, assumed a peculiar granular aspect, as 
if the mineral had been somewhat “crumbled ” zn situ. The micro- 
scopic structure fully confirms the conclusion, formed in the field, 
that after the rock had assumed its present mineral character, it 
was subjected to great lateral pressure, often crushing the minuter 
laminee of quartz as well as the larger veins of the same mineral, 
and abruptly bending the more yielding lamine of mica, which were 
now and then somewhat broken. 
The Black Mineral.—This is generally associated with the darker 
mica, occurring in minute specks, vermicules, and rods, forming fine 
black bands and streaks, which under a low power are often almost 
wholly opaque. Among the quartz occasional specks and rare larger 
grains occur ; but even in the latter anything like a definite crystal- 
line outline is wanting. Mr. J.J. H. Teall has kindly undertaken 
some investigations, experimenting with much care both on prepared 
slides and on a specimen of the rock powdered, with the results (i) that 
the colouring matter is principally iron-peroxide, but that there is 
probably a small amount of carbonaceous matter, and (ii) that the 
rock contains about 6 per cent. of combined water. Besides the 
above there is often a little iron-glance, limonite, and a grain or 
two which we may regard as impure epidote. 
Fifteen’ specimens have been examined, on which I will make a 
few remarks. One, from the first rock visible on the north of Hall- 
Sands village (p. 5), 2. e. from the first outcrop of true schist on 
the east coast, though much affected by crushing and decomposition 
(as its macroscopic aspect led me to expect), appears to me to bea 
true foliated rock. Another specimen, from near a fissure about 100 
yards further south, is a mica-schist, like several of those following, 
but extraordinarily crumpled. A specimen taken from a greenish 
band in this neighbourhood has, as we might expect, more the aspect 
of the slides of the chloritic schist presently to be described, but it 
contains some mica and a fair amount of calcite. It has, as is com- 
mon with the chloritic schists, a rather “ fragmental ” look ; but this 
I believe to be the result of subsequent crushing. 
