Wall and Sawkins are inclined to connect it with asphalt springs and pitch lakes.

'There is,' they say, 'easy gradation from the smaller Salses to the ordinary naphtha or petroleum springs.' It is certain that in the production of asphalt, carbonic acid, carburetted hydrogen, and water are given off.

'May not,' they ask, 'these orifices be the vents by which such gases escape?
And in forcing their way to the surface, is it not natural that the liquid asphalt and slimy water should be drawn up and expelled ?' They point out the fact, that wherever such volcanoes exist, asphalt or petroleum is found hard by.

The mud volcanoes of Turbaco, in New Granada, famous from Humboldt's description of them, lie in an asphaltic country.

They are much larger than those of Trinidad, the cones being, some of them, twenty feet high.