We walked upon granite mingled with siliceous tufa.

我们走在混有硅质凝灰岩的花岗岩上。

Under the changes of weather it may look like marble or like sea water, black as slate in the fog, white as tufa in sunlight.

在天气的变化下， 它可看起来大理或海水，在雾中一样黑， 在阳光下灰华一样白。

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.

冰岛完全没有冲积土，完全由火山凝华组成，也就是说，多孔岩和头的聚集体。