Geology Reference
In-Depth Information
Figure 2.16
Fine chilled glassy margin of a dyke cutting through a coarse
grained sill intrusion, Skye, Scotland.
In general, when describing a contact zone involving igneous rocks, measure
the orientation of the contact in two dimensions, and, where possible, in three
dimensions, to help you trace the feature from one outcrop to the next. Make
notes on the orientation, rock-types and geometry of veins, aplites, pegmatites
and chilled margins, all of which are closely related to physical processes occur-
ring at or near the contact of an igneous body with its host. Note also any other
features of contact zones, such as the occurrence of country rock inclusions
(xenoliths) and contact metamorphism in the surrounding rocks. Finally, if an
outcrop contains cross-cutting sets of joints, veins, dykes and other contacts,
then it can be immensely useful to establish a sequence of geological events by
examining their relationships (see examples in Chapter 6).
2.7 Summary of Igneous Outcrop Descriptions
The check list in Table 2.1 can be used as a guide to some of the key things to
be looking out for and to record when first describing outcrop scale features. It
requires the broader knowledge of detail contained within the rest of this topic
and should be used in conjunction with the check list for describing igneous
textures at the end of Chapter 3 (Table 3.6).
