File:The Quarterly journal of the Geological Society of London (14076434975).jpg

OLDER PERIDOTITES OF 8C0TLA^^D.
359
parallel to those displayed by the granites, and so well described by
Mr. J. A. Phillips, F.E.S.* Still more frequently the so-called
" segregation " veins of peridotite are seen traversing the gabbro,
and similar veins of gabbro are found intersecting the peridotite
masses. IS^ot unfrequently, too, such veins are found composed of
picrite, eucrite, or troctolite. That these so-called "segregation"
or " contemporaneous " veins have been formed at different times is
shown by the fact that one vein is frequently intersected and shifted
in position by another. A beautiful example of this kind is illus-
trated in the annexed diagram.
Intersecting Veins of Gabbro and Dunite in Olivine EocJc, ^ nat . size
a. Olivine rock (dunite).
b. Olivine-gabbro vein traversing the same.
c. Segregation-nodule of felspathie rock.
d. Porphyritic dunite intersecting and shifting the gabbro-vein.
As is not uncommonly the case with such veins, the minerals in
them have sometimes a special arrangement of their own. Thus in
the case figured above, the olivine-gabbro -vein (6) has its felspar
grouped principally along the sides, and the augite and olivine in the
centre.
The relations of the gabbros and peridotites in the Western Isles
of Scotland seem to indicate that in the heart of these old volcanoes
the felspar, olivine, and augite tended to segregate in certaia cases
into masses of various dimensions ; and that these masses were, after
consolidation, fissured again and again, the fissures being injected by
different portions of the magma, which were still in a more or less
plastic condition.
The very close association of peridotites with gabbros is not by
any means peculiar to the district we are describing. In the Hartz,
in ITassau, in Baden, in Silesia, and in many other districts a similar
association of rocks prevails : and in many, if not all of these cases.

• Quart. Journ. Geol. Soc. vol. xxxvi. (1880) p. 1.