Report on the scientific results of the voyage of during the years 1873-76 : under the command of Captain George SNares, , Captain Frank Turle Thomson, . rine chain of the South Atlantic. Theground rises to the height of the Islands of Gough and Tristan da Cunha, around which soundings of 1100 fathomsand upwards have been made. To the east of the islands the bottom sinks to 2200 fathoms, between long. 10° W and15° E., and from lat. 30° to 50° S. 76 THE VOYAGE OF CHALLENGER. Torrents and atmospheric erosion have worn gullies in these walls of rock, and h
Report on the scientific results of the voyage of during the years 1873-76 : under the command of Captain George SNares, , Captain Frank Turle Thomson, . rine chain of the South Atlantic. Theground rises to the height of the Islands of Gough and Tristan da Cunha, around which soundings of 1100 fathomsand upwards have been made. To the east of the islands the bottom sinks to 2200 fathoms, between long. 10° W and15° E., and from lat. 30° to 50° S. 76 THE VOYAGE OF CHALLENGER. Torrents and atmospheric erosion have worn gullies in these walls of rock, and heapedtogether piles of debris, which have accumulated to a height of 100 feet at the footof the cliffs. This circle of volcanic fragments is, in its turn, edged by a belt of gravelof the same nature, which is spread out on the narrow shore of the island. There is perhaps no region in the world where atmospheric agencies exert theirdestructive action in so energetic a manner as here. For nine months in the yearterrible tempests run riot on the island, and when the season of rains has ended, andthe snow that has accumulated on the top of the peak begins to melt, the water rushes. The Island of Tristan da Cunha. down in cascades, carrying an immense quantity of debris. These streams vigorouslyattack and demolish the less coherent and homogeneous of the layers that form thehorizontal strata ; they lay bare the rocks of the dykes, and cut deep indentations inthe ledge of the terrace. The transverse dykes alone resist the erosion, and stand uplike walls. Mr. Buchanan observes that at Tristan, as at Nightingale Island, the dykeshave, at their contact, made the volcanic breccia which they traverse more alterable ;whence it results that denudation acts by preference along their sides. These dykesof massive injected rocks also form the axis along which the coves and bends of the REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 77 shore are hollowed out. On the Island of Tristan the gully
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectscientificexpedition
