Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/513

Rh The highest of the perpendicular faces which bound it, rise about 60 or 70 feet above the high water mark, and these are on the southwestern side, where the most remarkable columns and where the great caves exist.

The greatest elevation of the island cannot be more than 120 feet above the level of the sea. There are no sunk rocks round it, but the water deepens rapidly from the shore, and admits of large vessels coasting it close at hand, provided they have a leading wind.

There is a soil of considerable depth on the surface, and it is covered with herbage.

It is almost superfluous to say that the whole island consists of a mass of basalt. I have indeed been told, that a sandstone bed has been seen at low water on the southwestern side, but I had not an opportunity of observing it. This is the part of the island, where, if in any place, it should, from the inclination of the strata, be perceived, and there is no reason to doubt the assertion, as we find most of the trap rocks of the Western Islands lying on beds of sandstone. It is equally superfluous to describe the basalt, since specimens of it are in every one's possession. It may be sufficient to remark, that its texture is more compact, more crystalline, and less earthy than that of basalt in general, and that it is at the same time less homogeneous, less black, more fragile, and more sonorous. But it would be idle to attempt to apply different terms to the endless varieties of the rocks of this tribe.

This basalt exhibits two modifications, the columnar, so often described, and the amorphous which is generally more or less amygdaloidal, containing imbedded zeolites of different sorts. I saw no examples of basaltic breccia, or of trap tuff, as it is improperly called.

It is in the amorphous basalt that the zeolites are most abundant. The nodules vary from the size of a pea to that of an hen's egg and