Research has shown igneous rock from the Devonian period (around 400 million years ago) intrudes into the surrounding metamorphic schists; the intrusions take the form of a series of concentric ring dikes. The innermost of these, known as the Inner Granite, constitutes the southern bulk of the mountain above , and also the neighbouring ridge of ; forms part of the Outer Granite, which is redder in colour. The summit dome itself, together with the steep northern cliffs, is composed of andesite and basaltic lavas.