of the remaining sixteen, some form promontories, and others stand at a little distance inland from the shore; but all have their southern sides either the lowest, or quite broken down.Two, however, of the sixteen had their northern sides also low, whilst their eastern and western sides were perfect.I did not see, or hear of, a single exception to the rule, of these craters being broken down or low on the side, which faces a point of the horizon between S.E.and S.W.This rule does not apply to craters composed of lava and scoriae.The explanation is simple: at this Archipelago, the waves from the trade-wind, and the swell propagated from the distant parts of the open ocean, coincide in direction (which is not the case in many parts of the Pacific), and with their united forces attack the southern sides of all the islands; and consequently the southern slope, even when entirely formed of hard basaltic rock, is invariably steeper than the northern slope.As the tuff-craters are composed of a soft material, and as probably all, or nearly all, have at some period stood immersed in the sea, we need not wonder that they should invariably exhibit on their exposed sides the effects of this great denuding power.Judging from the worn condition of many of these craters, it is probable that some have been entirely washed away.As there is no reason to suppose, that the craters formed of scoriae and lava were erupted whilst standing in the sea, we can see why the rule does not apply to them.At Ascension, it was shown that the mouths of the craters, which are there all of terrestrial origin, have been affected by the trade-wind; and this same power might here, also, aid in making the windward and exposed sides of some of the craters originally the lowest.
MINERALOGICAL COMPOSITION OF THE ROCKS.
In the northern islands, the basaltic lavas seem generally to contain more albite than they do in the southern half of the Archipelago; but almost all the streams contain some.The albite is not unfrequently associated with olivine.I did not observe in any specimen distinguishable crystals of hornblende or augite; I except the fused grains in the ejected fragments, and in the pinnacle of the little crater, above described.I did not meet with a single specimen of true trachyte; though some of the paler lavas, when abounding with large crystals of the harsh and glassy albite, resemble in some degree this rock; but in every case the basis fuses into a black enamel.Beds of ashes and far-ejected scoriae, as previously stated, are almost absent; nor did I see a fragment of obsidian or of pumice.Von Buch believes that the absence of pumice on Mount Etna is consequent on the feldspar being of the Labrador variety ("Description des Isles Canaries"page 328.); if the presence of pumice depends on the constitution of the feldspar, it is remarkable, that it should be absent in this archipelago, and abundant in the Cordillera of South America, in both of which regions the feldspar is of the albitic variety.Owing to the absence of ashes, and the general indecomposable character of the lava in this Archipelago, the islands are slowly clothed with a poor vegetation, and the scenery has a desolate and frightful aspect.
ELEVATION OF THE LAND.