The oldest stratified rock in Scotland is that called by Sir R.
Murchison "the fundamental gneiss," which is found in the north-west of Ross-
shire, and in Sutherlandshire (see Figure 82), and forms the whole of the
adjoining island of Lewis, in the Hebrides. It has a strike from north-west to
south-east, nearly at right angles to the metamorphic strata of the Grampians.
On this Laurentian gneiss, in parts of the western Highlands, the Lower Cambrian
and various metamorphic rocks rest unconformably. It seems highly probable that
this ancient gneiss of Scotland may correspond in date with part of the great
Laurentian group of North America.
CHAPTER XXVIII.
VOLCANIC ROCKS.
External Form, Structure, and Origin of Volcanic Mountains.
Cones and Craters.
Hypothesis of "Elevation Craters" considered.
Trap Rocks.
Name whence derived.
Minerals most abundant in Volcanic Rocks.
Table of the Analysis of Minerals in the Volcanic and Hypogene Rocks.
Similar Minerals in Meteorites.
Theory of Isomorphism.
Basaltic Rocks.
Trachytic Rocks.
Special Forms of Structure.
The columnar and globular Forms.
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