Geology Reference
In-Depth Information
(a)
(b)
(c)
Figure 7.16
Examples of magmatic layering in granites. (a) and (b) show cross-
cutting Schlieren layers with local megacryst accumulations, Sierra Nevada,
USA. (c) Regular and truncated biotite layers, Cape Granites, Cape Town, South
Africa.
and permeability of fractures in igneous rocks. They also act to weaken the
overall rock structure or rock mass. Ways to classify their effects in the field
are discussed in Chapter 10.
7.5.3 Aplites and pegmatites
Aplites and pegmatites are veins comprised mainly of quartz and alkali feldspar
with subordinate muscovite and occasional garnet that form during the final
stages of cooling and crystallisation (Figure 7.19). They are distinguished
