St. Kilda, with its attendant islets and stacs, is the
relic of what must once have been a much more extensive igneous
complex. no trace of effusive or sedimentary rocks, nor of
possible metamorphosed inclusions of such rocks, has been found.
Though stratigraphical evidence is wanting, it is possible on
lithological grounds to classify the complex as Tertiary in age,
the rock suite showing strong affinities with the deeper - seated
igneous rocks of other areas within the Brito- Icelandic Tertiary
The oldest rocks are a series of olivine-bearing and
olivine -free gabbros, by far the most abundant type being an
olivine eucrite carrying a small amount of hypersthene. On
account of the detached nature of the outcrops, the paucity of
inland exposures, and the physical difficulties encountered among
the cliff sections, it has not been possible to determine the
relations of one gabbro to another. i,ioreover, the absence of a base or summit to the complex prevents the determination of the
form taken by the individual intrusions.
The gabbros have been tñtruded, first by rather coarsegrained
olivine- and olivine -free dolentes in sheet-like form,
and later by a series of finer-grained dolerites, one of which,
exposed in the Village tray and on the northern cliffs of the
main island, is a basic olivine dolerite. The contact of the
finer -grained dolerites with one another and with the older
gabbros is of a puzzling nature, but they are regarded as forming
a sheet-like complex. On their contact with the gabbros
to the north of Lt. Kilda they appear to dip north -eastwards.
Un the western coast they dip south -westwards for the greater
part of their exposure, but at Laiiahrig nan Gall, where they
disappear under the eucrite of the W. Coast, they are seen dip-
The basic rocks are ántruded by three masses of grano-
-p jyre. '.she largest, and probably the youngest, of these in-
-trusions is a hornblende -bearing granophyre carrying about 74%
of silica and forming the northern third of the main island..
Its contact plane dips south -westwards at an average angle of
probably about 60°. The two remaining intrusions are sheet -like
in form and are exposed in the Glen bay to the north and among
the south -western cliffs of the :gun and the principal island.
c They dip south -westwards at high angles and have been slightly basïfied as the result of incorporation of basic rock. The in-
-trusion of the south -western coast is in the form of twó irregular
sheets running parallel to one another and separated only by
some twenty or thirty feet. Their contacts with the basic rocks,
which they have breccia,ted for a wide area, are very irregular.
The finer-grained dolerites have been pierced by a complicated
system of net -- veining proceeding from the granophyre intrusions
and hybrid rocks have been produced at several points as the
result of interaction between the granophyre magma and the sur*
-rounding basic rocks.
Crushing has been observed within the gabbros at many
points, and two types of crushing have been noted: 1st, crushing,
with the production of mylonitic rock without re-crystallisation
of the crushed minerals, and 2nd, crushing with the production
of a granulitised rock in which re- crystallisation has taken
place. The cataclastic phenomena appear to have originated
prior to the actual intrusion of the various granophyres, and it
is thought that earth -disturbances, perhaps the result of gas ex-
-plosions preceding the intrusion of the acid rocks, have pro - -duced lines of weakness along which the granophyres have subsequently
been intruded. The granophyre of trie Glen Bay
intrusion appears to have been slightly crushed after consolidation.
The majority of the dykes on the main island, the un,
and Soay strike 14.U. -- S.E., but on the east coast of Soay, on
boreray, and among the northern cliffs of the main island they
trend 1a.1'. - S.W. ureat difficulty has been experienced in de_
-terra.; IiinL; the respective ages of the different dykes, but a series of dolerite- basalt and porphyritic litchstone dykes (or
t}ieir ?devi.tr. i.f ied representatives) are definitely of Post - granophyre a e and have been found cutting all the other dyke
The rounded nature of the inland topography suggests
glacial action but in the absence of rock- striae, roches moutonpees,
moraines, or other glacial phenomena it is not possible to
give any account of the glaciation of St. 1;ilda. boulders of
gneiss discovered below the 100 -foot contour line in the Village
tray and the presence of garnets in the soil within the village
wall probably point to ice transport of material derived perhaps
from iarris or gist to the east. Iio trace of a raised beach
has been found among the islands.