Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/299

Rh long, and towards the north-east is often intersected by strata of porphyritic slate. These lavas are more earthy and cellular than those which I have had an opportunity of observing elsewhere, yet they contain fewer extraneous substances than those of Ætna and Vesuvius; they are in some places exposed to view in the vallies similar to those of the Corral in the island of Madeira. The valley of Las Guanchas on the north-west side of the Peak, contains according to M. Escolar above 100 strata of lava, the one reposing upon the other, at times alternating with pumice and tufa. The depth of these strata varies. M. Escolar has seen one of basaltic lava between 100 and 150 feet in depth in one solid mass, cellular at the surface, but gradually becoming more compact towards the bottom. This basaltic lava contains olivine and hornblende, and, in the caves on the coast, zeolite. This substance is also found in stalactites and in masses, sometimes in layers spread between the strata and diffused over the rock.

Nodules of chalcedony are sometimes also found, but these substances occur only in the chain of mountains towards the north-east, from the northern extremity of Santa Cruz to the point of Hidalgo.

The lavas of the island are of an endless variety, and the number of streams that have flowed are much beyond all enumeration. The whole surface is either ash, or solid or decomposed lava, which seems again and again to have been perforated by volcanic eruptions; the number of small extinct volcanoes is prodigious, they are to be found in all parts of the island, but the stream that has flowed from even the largest of them, such as the lava of the Peak called el Mal Pais, is trifling in comparison with that immense mass of lava mountains