69

are vesicular and amygdaloidal, and a sill on the north shore
of Triangle island contains vesicles so distributed as to indicate
their translation toward the top of the sill under the influence
of gravity, as shown in Figure 6. On the south side of Lina
island there is a composite sill of andesite, with later augite
andesite porphyrite occupying the same cavity. Aplitic stringers
up to an inch wide, irregularly disposed, and grading into the
rest of the rock, were found in a hornblende andesite dyke, a
rare type containing small phenocrysts of hornblende. The
aplitic veinlets are much less altered than the andesite contain-
ing them. The same dyke contains an irregular altered shale
xenolith about 3 feet in diameter. The severe heating which the
xenolith has undergone has not altered it appreciably and vir-
tually the only change it has suffered is a partial replacement
by pyrite. Other smaller xenoliths of shale are commonly
found near the margins of the dykes, where they have been stoped
from the walls.

Structure—External. The most striking external action
of the injected bodies is their baking action on the rocks they
intrude. A hardened, resistant zone is found bordering the
dykes and sills in almost every case, but there seems to be no
definite relation between the width of the zone and the thick-
ness of the intrusive, indicating a varying amount of superheat
in different dykes. The zone is up to 2 feet thick, and weathers
in relief, generally adhering strongly to the dyke walls. This is
illustrated in Plate XIIB. The actual contacts of the dykes
and sills with the shales are linear in the main, but remarkably
irregular in detail. A zone one-fourth to one-half inch in thick-
ness occurs paralleling the dyke, in which there is an intimate
interpenetration of intrusive and enclosing rock. This gives
the surfaces of the dyke walls an irregularly pimpled appearance
when exposed.

The sills are not in every case strictly concordant bodies,
some of them cut across the strata for short distances, and re-
turn to the bedding planes. In places a gap appears between
different portions of the same sill, as illustrated in Figure 7.
This is analogous to the dykes described and figured by Harker'.

1 Harker, A., ‘Tertiary Igneous Rocks of Skye,’’ 1904, p. 303.

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IEE OSEID GSE EDEMA FEO LRAT EDO AS AISLE
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