They are at a depth of about 12 ft., in slaty shale containing Llandeilo fossils and contemporaneous felspathic ash and scoriae.
In some instances the cones are quite intact, and the beds of ash and scoriae are as yet almost unaffected by denuding agencies.
The solid matter thrown out by volcanoes consists of lapilli, scoriae, dust and bombs.
Along these fissures, in a few days, scoriae cones appeared; on one occasion no less than thirty-six in number.
After the scoriae have been skimmed off, the upper and purer portion of the mass is refined, and the lower part re-melted.
The first of these shows no sign of either lavas or scoriae, but volcanic rocks occur on the margins of the other two.
We have as yet no proof that Man witnessed the origin of these hills of lava and scoriae of the middle phase of volcanic action.
The column of cinders, of scoriae, of broken rocks and earth, had wholly ceased to ascend.
It has been found profitable to resmelt the scoriae of the ancient workings.