We walked upon granite mingled with siliceous tufa.

我们走在混有硅质凝岗上。

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.

在天气变化下， 它可看起来像大理或海水，在雾中像板一样黑， 在阳光下像一样白。

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，完全由火山凝组成，也就是说，多孔和头聚集体。