The eastern and northern boundaries of Tobago consist of a series of basaltic rocks formed into mountains of considerable elevation. The heated matter in a fluid state must, therefore, have been stopped here, in its progress under the bed of the ocean, and has found its way to the surface, elevating at the same time, and modifying by its action, the earthy deposits through which it had to pass, and causing them to form acute or obtuse angles, according to their nearness or distance from the moving force. The mountain masses, however, of this now cooled and crystallized substance, show to what extent, even here, it burst forth, and has overflowed those previously superincumbent strata, while in a heated and liquid form.

As these elevations cooled down and became solid masses, they would naturally interrupt the current of the ocean, and, by causing eddies or stagnant pools, allow a new deposition of earthy particles to take place, which, in progress of time, have filled up the intervening spaces that now form the plains and valleys that so beautifully diversify the surface of this island. The tongue of low land that stretches to the southwest, and may in after-ages reach the shores of Trinidad, is a continuation of the alluvial deposits from the detritus of the earthy masses, as well as from the interrupted current of the sea.