File:The Quarterly journal of the Geological Society of London (13204584773).jpg

10
PROE. T. G. BONNEY ON THE HORNBLEND1C AND OTHER
diameter. Sir H. De la Beche mentions the occasional occurrence
of fragments of hornblende schist; but as he sometimes permits
himself (as was at that time inevitable) a little laxity in the use of
the term, I was anxious to settle a point obviously of the first
importance with relation to the age of the two series. After a
prolonged search, I at last succeeded in discovering an indubitable
fragment of hornblende schist, and so corroborating his statement.
The fragments are rare ; for it was not till my second visit, and
after searching for at least an hour, that I found it. The quartz-
felspar pebbles also clear. y come from a metamorphic series, if not
from the exact horizon that I have mentioned, so that there can be
henceforth no question that the two series are separated by an
enormous interval of time, and that there is no approach to a
passage of the slaty mudstones into the hornblende schist.
Section in Polurrian Cove,
a b c
a. Slate.
b. Breccia.
c. Schist, /. Fault.
Turning now to the junction of the slaty with the hornblendic
series in Polurrian Cove, on the western coast, we find similar
evidence of the complete separation of the two. The former
is a smooth, satiny, blackish or brownish slaty rock, veined
occasionally with quartz and calcite, in which are lenticular
sandy bands, dipping generally about 30° E.S.E. The horn-
blendic series is very massive, resembling at first sight a decom-
posed greenstone, but disclosing on closer examination indubitable
signs of foliation and bedding: it dips at about 70° S. At the
junction is a fault-breccia, a yard or two thick, composed mainly,
if not wholly, of fragments of the newer series in a rusty-
looking matrix*. The above diagram shows the general rela-
tions of the two series. The two rocks on opposite sides of the
fault are as different lithologically as is possible, the one being
almost unalteredf, the other as highly metamorphosed as is
possible. .

• This is described by Sir H. De la Beche, though with much hesitation, as
  

a conglomerate. I think that there is a. better exposure now than on the occa-
sion of his visits, and that there can be no doubt on the matter.
1" I believe there is a true cleavage, but correspondent with the bedding ; the
surfaces of this have been rumpled by subsequent lateral pressure, which has
left its mark on the microscopic structure of the rock. This would be termed a