The trap throughout the district consists mainly of augitic lavas, comprising the various forms of basalt. The compact and amorphous rock, with its semi-vitreous texture and its perfect conchoidal fracture, known specifically as basalt, occurs on the highest summits of the hills in large cubic masses. It is dark in the interior, containing small cavities coated generally with a reddish or black vitreous glaze, while the exterior is of a yellowish-brown colour.
