Volcanic Islands

第75章

There was only one appearance which at first made me doubt concerning the origin of the cast,--namely, that the finer roots from different stems sometimes became united together into upright plates or veins; but when the manner is borne in mind in which fine roots often fill up cracks in hard earth, and that these roots would decay and leave hollows, as well as the stems, there is no real difficulty in this case.Besides the calcareous branches from the Cape of Good Hope, I have seen casts, of exactly the same forms, from Madeira* and from Bermuda; at this latter place, the surrounding calcareous rocks, judging from the specimens collected by Lieutenant Nelson, are likewise similar, as is their subaerial formation.

Reflecting on the stratification of the deposit on Bald Head,--on the irregularly alternating layers of substalagmitic rock,--on the uniformly sized, and rounded particles, apparently of sea-shells and corals,--on the abundance of land-shells throughout the mass,--and finally, on the absolute resemblance of the calcareous casts, to the stumps, roots, and branches of that kind of vegetation, which would grow on sand-hillocks, I think there can be no reasonable doubt, notwithstanding the different opinion of some authors, that a true view of their origin has been here given.

*(Dr.J.Macaulay has fully described ("Edinb.New Phil.Journ." volume 29page 350) the casts from Madeira.He considers (differently from Mr.Smith of Jordan Hill) these bodies to be corals, and the calcareous deposit to be of subaqueous origin.His arguments chiefly rest (for his remarks on their structure are vague) on the great quantity of the calcareous matter, and on the casts containing animal matter, as shown by their evolving ammonia.Had Dr.Macaulay seen the enormous masses of rolled particles of shells and corals on the beach of Ascension, and especially on coral-reefs; and had he reflected on the effects of long-continued, gentle winds, in drifting up the finer particles, he would hardly have advanced the argument of quantity, which is seldom trustworthy in geology.If the calcareous matter has originated from disintegrated shells and corals, the presence of animal matter is what might have been expected.Mr.Anderson analysed for Dr.

Macaulay part of a cast, and he found it composed of:--Carbonate of lime......73.15

Silica.................11.90

Phosphate of lime.......8.81

Animal matter...........4.25

Sulphate of lime......a trace 98.11)

Calcareous deposits, like these of King George's Sound, are of vast extent on the Australian shores.Dr.Fitton remarks, that "recent calcareous breccia (by which term all these deposits are included) was found during Baudin's voyage, over a space of no less than twenty-five degrees of latitude and an equal extent of longitude, on the southern, western, and north-western coasts." (For ample details on this formation consult Dr.

Fitton "Appendix to Captain King's Voyage." Dr.Fitton is inclined to attribute a concretionary origin to the branching bodies: I may remark, that I have seen in beds of sand in La Plata cylindrical stems which no doubt thus originated; but they differed much in appearance from these at Bald Head, and the other places above specified.) It appears also from M.

Peron, with whose observations and opinions on the origin of the calcareous matter and branching casts mine entirely accord, that the deposit is generally much more continuous than near King George's Sound.At Swan River, Archdeacon Scott states that in one part it extends ten miles inland.("Proceedings of the Geolog.Soc." volume 1 page 320.) Captain Wickham, moreover, informs me that during his late survey of the western coast, the bottom of the sea, wherever the vessel anchored, was ascertained, by crowbars being let down, to consist of white calcareous matter.Hence it seems that along this coast, as at Bermuda and at Keeling Atoll, submarine and subaerial deposits are contemporaneously in process of formation, from the disintegration of marine organic bodies.The extent of these deposits, considering their origin, is very striking; and they can be compared in this respect only with the great coral-reefs of the Indian and Pacific Oceans.In other parts of the world, for instance in South America, there are SUPERFICIAL calcareous deposits of great extent, in which not a trace of organic structure is discoverable; these observations would lead to the inquiry, whether such deposits may not, also, have been formed from disintegrated shells and corals.

CAPE OF GOOD HOPE.

After the accounts given by Barrow, Carmichael, Basil Hall, and W.B.Clarke of the geology of this district, I shall confine myself to a few observations on the junction of the three principal formations.The fundamental rock is granite (In several places I observed in the granite, small dark-coloured balls, composed of minute scales of black mica in a tough basis.In another place, I found crystals of black schorl radiating from a common centre.Dr.Andrew Smith found, in the interior parts of the country, some beautiful specimens of granite, with silvery mica radiating or rather branching, like moss, from central points.At the Geological Society, there are specimens of granite with crystallised feldspar branching and radiating in like manner.), overlaid by clay-slate: the latter is generally hard, and glossy from containing minute scales of mica;it alternates with, and passes into, beds of slightly crystalline, feldspathic, slaty rock.This clay-slate is remarkable from being in some places (as on the Lion's Rump) decomposed, even to the depth of twenty feet, into a pale-coloured, sandstone-like rock, which has been mistaken, Ibelieve, by some observers, for a separate formation.I was guided by Dr.

Charles Darwin

作家的话

去QQ阅读支持我

还可在评论区与我互动

百万小说,新用户免费读

下载QQ阅读APP