Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/405

 lava contains minute pieces of olivine; sometimes it assumes a prismatic form, and in one place was of a moderate degree of hardness: the principal springs of water in the island issue from this stratum. On the top is the third, a greyish lava, generally compact, though at times near the surface very cellular, and containing much olivine. This lava takes principally the prismatic form of basalt. I have seen it in the most perfect prisms from 30 to 40 feet or more in height, the surface being covered with scoria, ash, and pumice. These masses of lava contain more or less, of what I consider to be olivine, occasionally carbonate of lime and zeolite, which last assumes either a crystallized or globular form, or is diffused in a thin coating between the different layers.

The fourth species of lava is of a coarse grain, is used for the making of walls, and the commonest and poorest houses are built of it, the blue and grey lavas being used for the copings, &c. It works easier than the two other kinds above-mentioned, is more friable and soft, and its colour is a mixture of brown and red. I observed it in a stratum by itself, and it did not seem to have any connection with the other three kinds.

These are the principal stratified lavas that the island affords, but in the beds of the rivers, particularly in that which flows in the valley of the Corral, several varieties occur in isolated masses, containing olivine and zeolite in greater or less quantity, and exhibing detached portions of strata, similar to those that are found in the Fossa Grande on the side of Vesuvius.

In the deep and singular valley called the Corral, which I had an opportunity of examing for several miles, the red and grey lava alternated five or six times. The tops of some of its barrier hills are formed of columnar basalt ; here and there rising to a peak, or broken into what might be termed a crystallised ridge, or tapering to a point