Indeed all the points of the peninsula are but remnants of a far larger sheet of land, which has been slowly eaten up by the surges of the gulf; which has perhaps actually sunk bodily beneath them, even as the remnant, I suspect, is sinking now.

We scrambled twenty feet down to the beach, and lay down, tired, under a low cliff, feathered with richest vegetation.

The pebbles on which we sat were some of pitch, some of hard sandstone, but most of them of brick; pale, dark, yellow, lavender, spotted, clouded, and half a dozen more delicate hues; some coarse, some fine as Samian ware; the rocks themselves were composed of an almost glassy substance, strangely jumbled, even intercalated now and then with soft sand.

This, we were told, is a bit of the porcellanite formation of Trinidad, curious to geologists, which reappears at several points in Erin, Trois, and Cedros, in the extreme south-western horn of the island.
How was it formed, and when?
That it was formed by the action of fire, any child would agree who had ever seen a brick-kiln.

It is simply clay and sand baked, and often almost vitrified into porcelain-jasper.