File:The Quarterly journal of the Geological Society of London (12981355314).jpg

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PEOCEEDINGS OF THE GEOLOGICAL SOCIETY.
.June 9,
These two analyses show the important bearing which chemical
composition has on jointing ; justifying the conclusion, so far as the
limestone of this neighbourhood is concerned, that the perfection of
the jointing is in proportion to the purity of the limestone.
Jointings in other rocks. — That the lithological character of the rock
is a matter affecting the nature of the divisional planes known under
the name of joints, is in many instances weU shown in rocks which
do not appertain to the Carboniferous series, and in which limestones
do not occur.
Among the arenaceous sandstones which constitute the Triassic
formation of Cumberland, the effect of a difference in mineral com-
position is in some localities well seen. One of these localities is at
the quarries of Howrig, near the Curthwaite Station on the Maryport
and Carlisle Eailway. Here we have thin-bedded red sandstones,
dipping 12°, N. by W. ; and these sandstones are generally intersected
by vertical joints, which have a N. and S. course. The joints in
each stratum commonly terminate with the bed. The strata of sand-
stone are either separated from each other by a thin parting of red
shale, or have more argillaceous matter entering into their composi-
tion near their bounding places. In either of these cases, the pre-
sence of this argillaceous matter produces an effect on the jointings,
rendering them less perfect and more indistinct in the more argilla-
ceous portions. The result of this change in lithological character
is represented in fig. 8.
Fig. 8. — Jointing in Triassic Sandstone, Howrig, Cumberland.
Sandstone
Shale.
The Silurian rocks of the south of Scotland, in some cases, exhibit
the effect of various mineral characters on jointing. In 'ciunnie
quarry, in the parish of Troqueer, in the Stewartry of Kirkcudbright,
about 2i miles west from the town of Dumfries, this is beautifully