Pegmatite, a form of granite with large crystals, breaks through layers of gneiss in summit of Tom Jones Mountain.  (Wyckoff 1971:17)

The Highlands exhibit many dikes.  Pegmatitie dikes, consisting of light-colored granitic rock with large crystals, are often seen cutting through layers of gneiss or granite.  Many are to be seen on Tom Jones Mountain.  (Wyckoff 1971:59)

A roche moutonnee (sheepback) on Tom Jones shows the typical ramp slope on north side, rugged slope on south side.  (Wyckoff 1971:65)