The stratification is gone; the porcellanite has run together into irregular masses, or fallen into them by the burning away of strata beneath; and the cracks in it are often lined with bubbled slag.
But whence carne the fire?
We must be wary about calling in the Deus e machina of a volcano.

There is no volcanic rock in the neighbourhood, nor anywhere in the island; and the porcellanite, says Mr.Wall, 'is identically the same with the substances produced immediately above or below seams of coal, which have taken fire, and burnt for a length of time.' There is lignite and other coaly matter enough in the rocks to have burnt like coal, if it had once been ignited; and the cause of ignition may be, as Mr.Wall suggests, the decomposition of pyrites, of which also there is enough around.

That the heat did not come from below, as volcanic heat would have done, is proved by the fact that the lignite beds underneath the porcellanite are unburnt.

We found asphalt under the porcellanite.

We found even one bit of red porcellanite with unburnt asphalt included in it.
May not this strange formation of natural brick and china-ware be of immense age--humanly, not geologically, speaking?
May it not be far older than the Pitch Lake above--older, possibly, than the formation of any asphalt at all?
And may not the asphalt mingled with it have been squeezed into it and round it, as it is being squeezed into and through the unburnt strata at so many points in Guapo, La Brea, Oropuche, and San Fernando?
At least, so it seemed to us, as we sat on the shore, waiting for the boat to take us round to La Brea, and drank in dreamily with our eyes the beauty of that strange lonely place.