File:The Quarterly journal of the Geological Society of London (12737030904).jpg

ASSOCIATED ROCKS OF THE LIZARD DISTRICT.
897
feet thick, forms a sort of terrace, some height above the shore,
which can be reached with a little difficulty ; the gabbro is extremely
coarse, the diallage crystals being often two or three inches long ;
and some of the largest occur in a vein only a few inches thick.
Just on the left there appears to be a small fault in the serpentine ;
on the right is a large included mass of schist forming a headland,
down to which descends a dyke of dark compact trap about a foot
wide. This can be seen to cut through two sheets of gabbro and the
serpentine between them, and ends abruptly against the schist. The
examination of this part of the coast leads to the following con-
clusions : —
1. That the serpentine is an intrusive rock.
2. That probably the hornblende schist was metamorphosed prior
to its intrusion.
3. That the gabbro was probably intruded when the serpentine
had arrived at its present condition.
4. That the black trap dyke was intruded last of all.
Under the microscope the trap is found to consist of a groundmass
generally microcrystalline, consisting probably of some pseudomorphic
Fig. 4. — Gabbro intrusive in Serpentine and Hornblende Schist
north of the Balk.
A. Serpentine.
B. Gabbro.
C. Gabbro showing a foliated structure.
D. Hornblende schist.
product after plagioclase, with perhaps minute crystals of actinolite
enlarged, and a large number of small hornblende crystals, merely
rather platy in structure, but varying from the normal to the actino-
litic form. There are, as usual, grains of magnetite, and a number of
microliths, sometimes acicular, sometimes rather irregular in form,
which are commonly included in the larger hornblende crystals, lying
with their longer axes in the planes of principal cleavage. I have
not been able to satisfy myself as to the nature of these. At pre-
sent we must call the rock a diorite ; but whether it has always been
hornblendic is by no means certain.
From the above headland a walk of about a quarter of a mile