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(a)
(b)
(c)
(d)
X
Y
Figure 6.8 A sequence of strata (a) showing the effect of intrusion of a locally
transgressive sill (b) or a completely concordant sill which is later displaced by
faulting (c). The position of the transgressive sill (b) is shown by the short dashed
lines in (a). In each case, the final disposition of the sill is similar (compare (b)
and (c)) but the displacement of strata above and below the sill and the possible
identification of deformation (e.g. fault brecciation) along the fault plane enables
(c) to be distinguished from (b). (d) Sections of sills that have been emplaced into
faulted strata. X is a sill that has been emplaced across the fault plane, whereas
Y is a sill emplaced at a particular stratigraphic horizon and which exhibits
transgression at the fault plane.
of an early intrusion of basic magma that has been followed by an intrusion of
acid magma is given in Figure 6.9; the contact between the two types varies
between very sharp and completely diffuse. In composite intrusions, the basic
and acid components might contain xenoliths or xenocrysts of each other. Homo-
geneous minor intrusions might contain inclusions of contrasted composition, for
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