We walked upon granite mingled with siliceous tufa.

我们走在混有硅质的花岗上。

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.

在天气的变化下， 它可看起来像大理石或海水，在雾中像石板黑， 在阳光下像石白。

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，完全由火山组成，也就是说，多孔石和石头的聚集体。