第15章

Brayley in having given me the following references to papers on this subject: Faraday in the "Edinburgh New Philosophical Journal" volume 15page 398; Gay-Lussac in "Annales de Chem.et Phys." tome 63 page 219translated in the "London and Edinburgh Philosophical Magazine" volume 10page 496.) Carbonate of lime can be heated to almost any degree, according to Faraday, in an atmosphere of carbonic acid gas, without being decomposed; and Gay-Lussac found that fragments of limestone, placed in a tube and heated to a degree, not sufficient by itself to cause their decomposition, yet immediately evolved their carbonic acid, when a stream of common air or steam was passed over them: Gay-Lussac attributes this to the mechanical displacement of the nascent carbonic acid gas.The calcareous matter beneath the lava, and especially that forming the crystalline spicula between the interstices of the scoriae, although heated in an atmosphere probably composed chiefly of steam, could not have been subjected to the effects of a passing stream; and hence it is, perhaps, that they have retained their carbonic acid, under a small amount of pressure.

The fragments of scoriae, embedded in the crystalline calcareous basis, are of a jet black colour, with a glossy fracture like pitchstone.Their surfaces, however, are coated with a layer of a reddish-orange, translucent substance, which can easily be scratched with a knife; hence they appear as if overlaid by a thin layer of rosin.Some of the smaller fragments are partially changed throughout into this substance: a change which appears quite different from ordinary decomposition.At the Galapagos Archipelago (as will be described in a future chapter), great beds are formed of volcanic ashes and particles of scoriae, which have undergone a closely similar change.

THE EXTENT AND HORIZONTALITY OF THE CALCAREOUS STRATUM.

(FIGURE 2: SIGNAL POST HILL.(Section with A low and C high.)A.--Ancient volcanic rocks.

B.--Calcareous stratum.

C.--Upper basaltic lava.)

The upper line of surface of the calcareous stratum, which is so conspicuous from being quite white and so nearly horizontal, ranges for miles along the coast, at the height of about sixty feet above the sea.The sheet of basalt, by which it is capped, is on an average eighty feet in thickness.Westward of Porto Praya beyond Red Hill, the white stratum with the superincumbent basalt is covered up by more recent streams.Northward of Signal Post Hill, I could follow it with my eye, trending away for several miles along the sea cliffs.The distance thus observed is about seven miles; but I cannot doubt from its regularity that it extends much farther.In some ravines at right angles to the coast, it is seen gently dipping towards the sea, probably with the same inclination as when deposited round the ancient shores of the island.I found only one inland section, namely, at the base of the hill marked A, where, at the height of some hundred feet, this bed was exposed; it here rested on the usual compact augitic rock associated with wacke, and was covered by the widespread sheet of modern basaltic lava.Some exceptions occur to the horizontality of the white stratum: at Quail Island, its upper surface is only forty feet above the level of the sea; here also the capping of lava is only between twelve and fifteen feet in thickness; on the other hand, at the north-east side of Porto Praya harbour, the calcareous stratum, as well as the rock on which it rests, attain a height above the average level: the inequality of level in these two cases is not, as I believe, owing to unequal elevation, but to original irregularities at the bottom of the sea.

Of this fact, at Quail Island, there was clear evidence in the calcareous deposit being in one part of much greater than the average thickness, and in another part being entirely absent; in this latter case, the modern basaltic lavas rested directly on those of more ancient origin.