John Parkinson— Rocks of Northern Guernsey. 7D) 
between St. Peter’s Port and St. Sampson’s, which is composed of 
hornblende gabbro. 
In the present paper the field evidence will be briefly reviewed, 
and reference made to the microscopic work only when necessary. 
The subject can be divided into two parts— 
(1) The hornblende gabbro. 
(2) Sections on the northern and western coasts. 
(1) THe Hornsienve GaBsRo. 
Detailed examination of the coast between St. Sampson’s and 
St. Peter’s Port resulted in the following conclusions :— 
(a) A hornblende-labradorite rock, the former mineral more or less. 
completely replacing augite, is characteristic. The rounded outlines 
of the ferromagnesian mineral give a spotted appearance to this rock, 
which may be considered as typical of the locality (‘ bird’s-eye’ of 
quarrymen). It varies remarkably, however, in the relative 
quantity of the distinctive minerals present, i.e., either felspar or 
ferromagnesian constituents may predominate ; such alteration taking 
place, sometimes as a gradual change, sometimes as a more rapid 
alternation of parts, of the nature of banding, in which the constituents. 
are equally distributed.'| A common variation results in a fine- 
textured rock, more basic than the average, in which the hornblende 
is original and not secondary, elongated prisms predominating instead 
of a rounded poecilitic form of augite; and the mineral, as seen in 
a thin section, is almost opaque through the presence of opacite. 
A banded structure, the occurrence of less clear and well-defined 
streaks differing mineralogically from the surrounding rock, and the 
parallel arrangement of such a mineral as hornblende indicate fluxional 
movement, and give rise to the irregular approximation of varieties. 
differing considerably in composition. 
(6) Later dykes cut these rocks, and belong to at least two periods. 
The earlier are rich in hornblende; the later, often very abundant, 
consist almost entirely of felspar. 
A thin section of the former shows that hornblende composes at. 
least half the rock. It forms polygonal grains (average size 
‘009 inch, but frequently considerably larger, say -03 inch), which 
approach to an ophitie structure through the earlier consolidation of 
the felspars, but have a tendency to gather into groups of grains to the 
entire exclusion of this mineral. 
The more acid dykes show a strong tendency to follow the directions. 
taken by the earlier system, with the result that a kind of composite 
dyke is produced. The variations in the hornblende gabbro go to show 
that a rock which forms a dyke in one place may in another pass 
gradually into one on the whole older which surrounds it. This 
close relationship is strongly borne in upon the observer. specially 
noteworthy are the large elongated hornblendes in the felspar dykes, 
producing a close resemblance to the extreme felspathic variation of 
the ‘bird’s-eye.’ The dimensions of the dykes exercise no influence 
on the dimensions of the hornblende. 
' See Professor Bonney’s description: Quart. Journ. Geol. Soc., yol. xl (1884), 
p- 425. Mica is rare in these rocks. ; 
