Geology Reference
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deposited crystals such as quartz, feldspar and zeolites. These are interpreted as
vapour cavities, analogous to amygdales and are termed druses or drusy cavities-
miarolitic cavities . Rocks containing such cavities are termed miarolitic rocks.
The presence of drusy cavities indicates exsolution of a fluid phase under low
confining pressure and hence provides evidence of crystallisation shallower than
a depth of about 1 - 2 km.
High-level intrusions may contain the range of xenoliths and inclusions
described in Section 2.4.3 and these are sometimes more conspicuous near the
upper margins than in deeper-level intrusions. In particular, high-level intrusions
often contain sharp-sided angular xenoliths which are absent in deep-level plu-
tonic intrusions. It is often difficult to establish the type and degree of connection
of high-level intrusions with the surrounding volcanic rocks, and therefore care-
ful study should be made of geological features such as xenoliths, inclusions
and contact characteristics that might contribute to the establishment of such
relationships. Finally it is possible to characterise the overall shape of the intru-
sions on a log-log length vs thickness diagram (Figure 6.14) which shows a
characteristic S shape.
Figure 6.14 Relationship of length with thickness for different scales of intru-
sion. Deviations from the a
=
1 line show asymmetric shapes of the intrusions
(i.e. not balloon shape).
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