Iceland, which is entirely devoid of alluvial soil, is wholly composed of volcanic tufa, that is to say, an agglomeration of porous rocks and stones. Before the volcanoes broke out it consisted of trap rocks slowly upraised to the level of the sea by the action of central forces. The internal fires had not yet forced their way through.

But at a later period a wide chasm formed diagonally from southwest to northeast, through which was gradually forced out the trachyte which was to form a mountain chain. No violence accompanied this change; the matter thrown out was in vast quantities, and the liquid material oozing out from the abysses of the earth slowly spread in extensive plains or in hillocky masses. To this period belong the feldspar, syenites, and porphyries.