John Parkinson—Rocks of Northern Guernsey. 77 
with a more translucent border, a little quartz, a considerable quantity 
of brown mica in flakes up to ‘028 inch across, and green hornblende 
taking the form of rounded polygonal grains. Further observation 
shows that the younger of the two varies greatly in composition, and, 
although usually containing hornblende and conspicuous crystals of 
biotite, yet the dark minerals may be locally absent or represented by 
a group of irregularly disposed and elongated hornblendes, always 
considerably larger than any of the other constituents. Naturally there 
are many minor yariations. For example, a slab may consist of two 
slightly different rocks, distinguished principally by their relative 
amounts of hornblende; the less basic containing drawn-out patches 
of a finer textured and darker rock. Fluxional movement is indicated 
by bands of different mineral composition, which occasionally are 
traceable for eight or ten feet. The almost invariable presence of 
quartz and the common occurrence of large ragged plates of biotite 
distinguish these rocks at once from the hornblende gabbro of the east 
coast. 
(6) The East Coast North of the Hornblende Gabbro.—At Hommet 
Bennest, to the north of Bordeaux Harbour, we find a close-grained 
diorite weathering grey through the presence of felspar,. which 
predominates slightly over the hornblende. ‘his rock is veined and 
streaked by one still more felspathic, which contains a ferromagnesian 
mineral, The older of the two rocks consists of a felspar too opaque 
for determination, slightly earlier in consolidation than the hornblende, 
an altered mica, and a little magnetite. Small patches of quartz and 
a translucent felspar are found here and there. Hornblende bordering 
these patches assumes the form of an actinolitic fringe. Comparison 
with other slides suggests these patches are the early stages of 
a permeation by a more acid magma. 
(e) From Vale Castle to Fort Norman.—A uniform blue-coloured 
quartz-diorite with no clear signs of veining or intrusion is quarried 
along the shore to the north of Vale Castle. This is a variety of the 
aiden rock of Grande Havre Quarry—a quartz-mica diorite. We 
meet with a very similar rock to the west of Lancresse Common, but 
differimg in being richer in quartz and biotite. In both slides the 
felspars are often zoned, the outer border more translucent than the 
centre, while the higher extinction angle and irregular outline of 
the central part suggests the corrosion of a basic species. A little 
more than half-way between the Castle and the Fort the diorite is 
eut by a lighter-coloured and coarser rock, containing, however, some 
quantity of hornblende, which locally assumes an elongated outline. 
The character of the intrusion is irregular and streaky. Ona jutting 
headland to the south of the Fort is a fine-grained diorite like that 
of Hommet Bennest, consisting of hornblende and felspar, the latter 
slightly the earlier in crystallization, some is untwinned, but plagio- 
clases occur varying from oligoclase to labradorite. It is broken 
by veins or dykes of a felspathic rock speckled with hornblende. 
Occasionally dykes, about an inch wide, can be traced for five or 
six feet. A thin section across the junction exhibits the following 
association: a fine-grained quartz-free hornblende plagioclase rock 
on the one side, a coarser quartz hornblende felspar rock on the other. 
