78 John Parkinson—Rocks of Northern Guernsey. 
The latter is the intruder, but the boundary between the two is quite 
indefinite, partly no doubt owing to the opaque state of the felspars, 
which hinders a precise determination. In the older rocks the horn- 
blende formed after the plagioclase, and is locally replaced by biotite ; 
an the younger the quartz is inconspicuous, and plagioclase is plentiful 
(oligoclase to labradorite), with a tendency towards idiomorphism. 
A striking contrast exists between this rock and the intrusive 
member at Grande Havre. Not only is the latter coarser, but its 
plentiful quartz and biotite distinguish it at once from any rock on 
the east coast, while the former is not unlike the late felspathic 
dykes cutting the hornblende gabbro. 
(d) East and South of Kort Marchant.—At the southern end of the 
Fort Marchant peninsula the diorite contains rounded hornblendes, 
resembling the spots on a trout, with which is associated a variety 
containing the same mineral as elongated prisms. Both are veined by 
a third rock, richer in felspar (some orthoclase and acid plagioclases), 
and containing a little quartz, some biotite, and green hornblende. 
This assemblage, for the members cannot well be separated, passes 
into a rock which, although poorly developed, is identical with the 
‘bird’s-eye’ of the shore to the north of St. Peter’s Port. It contains 
segregations of hornblende and locally more felspathic patches con- 
taining hornblendes, connected in some instances with irregular veins. 
Similar phenomena of variation, and intrusion which may be called 
intermingling, are abundant elsewhere in the northern part of the 
island. Thus we find evidences of a brecciation more or less sharply 
defined all over the quarries near the Saumerez Monument. 
The younger rock is of a more granitic composition than that into 
which it has forced its way, and when pure, 1.e. unassociated with 
fragments, seemingly not rich in either black mica or hornblende. 
The older rock is a quartz-diorite, on the whole uniform in texture, 
but which contains more basic fragments, having edges sometimes well 
defined, sometimes passing into the surrounding rock in a cloudy way, 
a zone half-an-inch or so wide being common to both. 
Locally the invading rock is distinctly streaky; the whole 
suggesting that the older member was in a state of imperfect con- 
solidation at the time of the intrusion of the more acid magma, and 
that some absorption took place. 
The hornblende-felspar rocks south of Fort Norman and from 
Hommet Bennest are essentially the same as the more basic parts 
of rocks from the west of Fort Doyle and the black dykes cutting the 
‘bird’s-eye’ of the St. Sampson’s area. 
Conclusions. 
1. That in the northern half of Guernsey we have a series of 
igneous rocks, which, with the probable exception of some granites 
on the west coast, are related to each other as the products of 
differentiation of a single magma. 
2. Some evidence exists for a progressive increuse in acidity in going 
north and west. 
