Some of the craters surmounting the larger islands are of immense size, and they rise to a height of between three and four thousand feet.

Their flanks are studded by innumerable smaller orifices.

I scarcely hesitate to affirm that there must be in the whole archipelago at least two thousand craters.

These consist either of lava and scoriae, or of finely-stratified, sandstone-like tuff.

Most of the latter are beautifully symmetrical; they owe their origin to eruptions of volcanic mud without any lava: it is a remarkable circumstance that every one of the twenty-eight tuff-craters which were examined had their southern sides either much lower than the other sides, or quite broken down and removed.