Difficulty in ascertaining the precise Age of a Plutonic Rock. — Test
of Age by Relative Position. — Test by Intrusion and Alteration. — Test
by Mineral Composition. — Test by included Fragments. — Recent and
Pliocene Plutonic Rocks, why invisible. — Miocene Syenite of the Isle
of Skye. — Eocene Plutonic Rocks in the Andes. — Granite altering
Cretaceous Rocks. — Granite altering Lias in the Alps and in Skye. —
Granite of Dartmoor altering Carboniferous Strata. — Granite of the Old
Red Sandstone Period. — Syenite altering Silurian Strata in Norway. —
Blending of the same with Gneiss. — Most ancient Plutonic Rocks. —
Granite protruded in a solid Form.