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OF SOME ROCKS FROM GUERNSEY. 427 
talline structure, consisting of elongated crystals (rather microlithic 
in habit) of oligoclase associated with a very brown mica, and a 
third mineral much altered and almost opaque ; ferrite and opacite 
are scattered about the slide. This third mineral, more highly 
magnified, appears to consist of a mixture of minerals, chloritic 
and ferruginous. It is not impossibly a replacement of a pyro- 
xenic mineral. Barrois notices the presence of gedrite in kersan- 
tite, a mineral whose chemical composition would be not unlikely 
to give rise to the appearances here described. 
40 (Rocquaine Castle)—This rock consists of subangular frag- 
ments (generally less than :01" diameter), among which a clear 
quartz predominates, scattered in a ground-mass, which appears 
to be composed of quartz, felspar, and a green filmy mineral, pos- 
sibly a chlorite, but more probably, I think, a variety of hornblende. 
This, I think, has certainly been formed in situ, and the ground- 
mass generally appears to have undergone some alteration. The 
rock has the aspect of a slightly altered sedimentary rock. 
AT (Rocquaine Castle, ** pocket” of coarse rock im finer).—In the 
greater part of the slide it has the appearance of a magnesia-mica 
_ gneiss (such as have been already described), which has evidently 
been considerably crushed, and here and there becomes quite pul- 
verized,.assuming an appearance which reminds us of No. 40. 
45 (Fort Doyle).—A most perplexing rock: nearly half the slide, 
with a small portion of the exterior of the remainder, appears to be 
a diorite or a coarse hornblendic gneiss, poor in quartz (consisting 
mainly of a plagioclastic felspar and hornblende), indicating consider- 
able crushing 7m situ; but between these there is a zone about 7 inch 
wide presenting a singular resemblance to a schist or schistose rock 
not highly altered, consisting of minutely granular quartzose and 
felspathic materials, associated with filmy scales of a greenish mica- 
ceous mineral, in which are rarely scattered a few rather larger 
grains of quartz or felspar. 
These three specimens are most perplexing. Taking 40 by itself, 
T came to the conclusion, though not without suspicion, that it was 
probably a slightly altered rock of sedimentary origin. As regards 
47, had I taken it alone, I should have been of opinion that it was 
almost certainly one of the coarser gneisses crushed im situ. 45 looks 
like an interbanding of a minutely constituted not very highly meta- 
morphosed schist in association with a coarse felspar-hornblende 
gneiss, which has been rather crushed. But is this association 
‘possible? It would be rash to give an absolute denial, with 
our present very limited knowledge of the metamorphic rocks and 
the effects of the agents of metamorphism, and I once collected 
specimens, near the top of the Bernina Pass, in Switzerland, where 
a coarse gneiss and a rather compact green schist appeared to occur 
in true association ; but in every other case that I have seen where 
finer and coarser foliated rocks are associated, the constituents of 
the finer under the microscope exhibit a more complete crystalliza- 
tion than is seen in the Guernsey specimens, and even in the instance 
at the Bernina (which was rather imperfectly exposed) the green 
