CHAPTER III.
THE SPREAD OF GLACIERS DURING COLD EPOCHS.
I have before explained that the conditions are such that the cold periods of the northern and southern hemispheres were concurrent. Through this cause, while the glacial epoch was being perfected, the ice followed down the mountain ranges of both hemispheres; and, while gathering on the lands of the temperate latitudes, it also spread over a portion of the tropical zone. It is reported that traces of ancient glaciers are found in India, and also in Central America and in tropical South America. In fact, the denudation caused by ancient glaciers on the elevated lands of the tropics are too well defined to be attributed to any process of weathering, while Alpine plants of the same species are found near the summits of mountains in the tropics as well as in the high latitudes of both hemispheres.
This fact goes to show that a portion of the lowlands of the tropical zone have experienced a temperature favorable for the growth of Alpine plants. And, judging from the tropical islands I have visited, situated in the cold currents which flow down the eastern sides of the oceans from the high latitudes, I think they show strong traces of having during some remote period been subject to the action of glaciers. The island of St. Helena, situated in the southern tropical Atlantic, has the appearance of having been heavily iced during a frigid age. Its steep ravines, which deepen as they approach the sea, recall to the southern voyager the ice-worn islands of the high latitudes. It seems improbable that these deep ravines which penetrate the hard volcanic rock, on their short course to the sea, could have been caused by their scanty brooklets.
The bowlders scattered over the island are not in harmony with the weathering process, while the obliteration of its craters seems to point to a more rapid process of erosion than could be attributed to weathering.
Professor Agassiz, in his “General Sketch of the Expedition of the ‘Albatross,’” states that the Galapagos Islands are of volcanic origin, and that their age does not reach beyond the earliest Tertiary period; and his report seems to favor the impression of their having undergone denudation sufficient to slough off large portions of the rims of the older craters, and also the eastern face of Wenman Island. On Hood’s Island, at the time of my visit, its crater had entirely disappeared.
The highest portion of the island, which was the probable site of its ancient crater, showed no trace of its former existence; yet at the foot of this low mountain, on its southern side, I saw a large collection of loose bowlders, composed of hard volcanic rock, which were mostly free from soil and other débris, and easily moved from their places, while the spaces afforded by the loose piles of dark basaltic rocks afforded a secure retreat for numerous owls and lizards. Beyond the rocky piles to the southward a horizontal area of land was strewn with bowlders to the sea, which was some two miles distant from the higher land. The bowlders which covered the plain were somewhat smaller than those at the foot of the mountain, as none of the former were more than three or four feet in their longest measurement.
They seem to have been formed from thin strata of lava, which were broken in pieces from pressure, such as the action of ice could perform. In fact, the crowded and angular and somewhat worn blocks of lava presented a different appearance from stones thrown from the crater of a volcano, while no such bowlders are found among the recent volcanic eruptions on the islands.
The plain so thickly strewn with bowlders, and partly shaded by a tall growth of shrubs, fell off abruptly at the seaside, forming a steep cliff some two hundred feet in height.
The rocky floor at the foot of the cliff received such débris as fell from the sea-washed land; yet it contained few bowlders, they having been washed away by the waves soon after falling.