I made the passage of the Cordilleras to Mendoza, the capital of the republic of that name, on horseback. The features in the scenery of the Andes which struck me most were that all the main valleys have on both sides a fringe, sometimes expanding into a narrow plain of shingle and sand. I am convinced that these shingle terraces were accumulated during the gradual elevation of the Cordilleras by the torrents delivering at successive levels their detritus on the beach-heads of long, narrow arms of the sea, first high up the valleys, then lower down and lower down as the land slowly rose.
If this be so, and I cannot doubt it, the grand and broken chain of the Cordilleras, instead of having been suddenly thrown up—as was till lately the universal, and still is the common, opinion of geologists—has been slowly upheaved in mass in the same gradual manner as the coasts of the Atlantic and Pacific have arisen within the recent period. The other striking features of the Cordilleras were the bright colours, chiefly red and purple, of the utterly bare and precipitous hills of porphyry; the grand and continuous wall-like dikes; the plainly divided strata, which, where nearly vertical, formed the picturesque and wild central pinnacles, but where less inclined composed the great massive mountains on the outskirts of the range; and lastly, the smooth, conical piles of fine and brightly-coloured detritus, which slope up sometimes to a height of more than 2,000 feet.
It is an old story, but not less wonderful, to see shells which were once crawling at the bottom of the sea now standing nearly 14,000 feet above its level. But there must have been a subsidence of several thousand feet as well as the ensuing elevation. Daily it is forced home on the mind of the geologist that nothing, not even the wind that blows, is so unstable as the level of the crust of the earth.
From Valparaiso to Coquimbo, and thence to Copiapo, in Northern Chile, the country is singularly broken and barren. On some of the terraced plains rising to the Cordilleras, covered with cacti, there were large herds of llamas. At one point in the coast range great prostrate silicified trunks of fir trees were very numerous, embedded in a conglomerate. I discovered convincing proof that this part of the continent of South America has been elevated near the coast from 400 feet to 1,300 feet since the epoch of existing shells; and further inland the rise possibly may have been greater. From the evidence of ruins of Indian villages at very great altitude, now absolutely barren, and some fossil human relics, man must have inhabited South America for an immensely long period.
From the port of Iquique, in Peru, a visit was made across the desert to the nitrate of soda mines. The nitrate stratum, between two and three feet thick, lies close to the surface, and follows for 150 miles the margin of the plain. From the troubled state of the country, I saw very little of the rest of Peru.
A month was spent in the Galapagos Archipelago—a group of volcanic islands situated on the Equator between 500 and 600 miles westward of the coast of America. The little archipelago is a little world within itself. Hence, both in time and space, we seemed to be brought somewhere near to that great fact, that mystery of mysteries, the first appearance of new beings on this earth. The vegetation is scanty. The principal animals are the giant tortoises, so large that it requires six or eight men to lift one. The most remarkable feature of the natural history of this archipelago is that the different islands are inhabited by different kinds of tortoises; and so with the birds, insects, and plants. One is astonished at the amount of creative force, if such an expression may be used, displayed on these small, barren, and rocky islands, and still more so at its diverse, yet analogous, action on points so near each other.
V.—The Coral Islands of the Indian Ocean
Having completed the survey of the coasts and islands of the South American continent, the Beagle sailed across the wide Pacific to Tahiti, New Zealand, and Australia, in order to carry out the chain of chronometrical measurements round the world. From Australasia a run was then made for Keeling or Cocos Island in the Indian Ocean. This lonely island, 600 miles from the coast of Sumatra, is an atoll, or lagoon island. The land is entirely composed of fragments of coral.
There is, to my mind, much grandeur in the view of the outer shores of these lagoon islands. The ocean, throwing its waters over the broad barrier-like reef, appears an invincible, all-powerful enemy. Yet these low, insignificant coral islets stand and are victorious; for here another power, as an antagonist, takes part in the contest. Organic forces separate the atoms of carbonate of lime, one by one, from the foaming breakers, and unite them in a symmetrical structure. Let the hurricane tear up its thousand huge fragments, yet what will that tell against the accumulated labour of myriads of architects at work night and day, month after month?
There are three great classes of coral reefs—atoll, barrier, and fringing. Now, the utmost depth at which corals can construct reefs is between twenty and thirty fathoms, so that wherever there is an atoll a foundation must have originally existed within a depth of from twenty to thirty fathoms from the surface. The coral formation is raised only to that height to which the waves can throw up fragments and the winds pile up sand. The foundation, such as a mountain peak, therefore, must have sunk to the required level, and not have been raised, as has hitherto been generally supposed.