第61章

Proofs of the rising of the land are scanty and imperfect.At Chatham Island, I noticed some great blocks of lava, cemented by calcareous matter, containing recent shells; but they occurred at the height of only a few feet above high-water mark.One of the officers gave me some fragments of shells, which he found embedded several hundred feet above the sea, in the tuff of two craters, distant from each other.It is possible, that these fragments may have been carried up to their present height in an eruption of mud; but as, in one instance, they were associated with broken oyster-shells, almost forming a layer, it is more probable that the tuff was uplifted with the shells in mass.The specimens are so imperfect that they can be recognised only as belonging to recent marine genera.On Charles Island, I observed a line of great rounded blocks, piled on the summit of a vertical cliff, at the height of fifteen feet above the line, where the sea now acts during the heaviest gales.This appeared, at first, good evidence in favour of the elevation of the land; but it was quite deceptive, for Iafterwards saw on an adjoining part of this same coast, and heard from eye-witnesses, that wherever a recent stream of lava forms a smooth inclined plane, entering the sea, the waves during gales have the power of ROLLINGUP ROUNDED blocks to a great height, above the line of their ordinary action.As the little cliff in the foregoing case is formed by a stream of lava, which, before being worn back, must have entered the sea with a gently sloping surface, it is possible or rather it is probable, that the rounded boulders, now lying on its summit, are merely the remnants of those which had been ROLLED UP during storms to their present height.

DIRECTION OF THE FISSURES OF ERUPTION.

The volcanic orifices in this group cannot be considered as indiscriminately scattered.Three great craters on Albermarle Island form a well-marked line, extending N.W.by N.and S.E.by S.Narborough Island, and the great crater on the rectangular projection of Albemarle Island, form a second parallel line.To the east, Hood's Island, and the islands and rocks between it and James Island, form another nearly parallel line, which, when prolonged, includes Culpepper and Wenman Islands, lying seventy miles to the north.The other islands lying further eastward, form a less regular fourth line.Several of these islands, and the vents on Albemarle Island, are so placed, that they likewise fall on a set of rudely parallel lines, intersecting the former lines at right angles; so that the principal craters appear to lie on the points where two sets of fissures cross each other.The islands themselves, with the exception of Albemarle Island, are not elongated in the same direction with the lines on which they stand.The direction of these islands is nearly the same with that which prevails in so remarkable a manner in the numerous archipelagoes of the great Pacific Ocean.Finally, I may remark, that amongst the Galapagos Islands there is no one dominant vent much higher than all the others, as may be observed in many volcanic archipelagoes: the highest is the great mound on the south-western extremity of Albemarle Island, which exceeds by barely a thousand feet several other neighbouring craters.