Charles Darwin's Works: Geological observations on the volcanic islands and parts of South AmericaD. Appleton, 1896 |
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albite andesite angle appears augite basaltic beds calcareous calcareous matter carbonate of lime Chile clay-slate cleavage cliffs coast colour compact composed consist Copiapo Coquimbo Cordillera crater Cruz crystalline crystals d'Orbig d'Orbigny denudation deposit described dikes distance earthy elevation embedded eruption escarpment feet in height feet in thickness feldspar feldspathic foliation formed fossils fragments Geological gneiss granite gravel gypseous formation gypsum hills horizontal hornblende inch inclined island laminated land lava layers limestone lower Macrauchenia mass metamorphic mica miles mountains nearly numerous obsidian origin Pampean Pampean formation parallel Patagonia pebbles Pecten plain porphyritic conglomerate porphyry Port Desire probably quartz range remarkable resembles ridge rocks round sand sandstone seen shells side Sowerby species specimens strata stratified streams structure surface terraces tertiary testá Tierra del Fuego tion tosca-rock trachyte tuff Turritella upper upraised Uspallata valley varieties veins volcanic Voyage
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Seite 303 - Iquique, though they stand about 3,000 feet above the level of the sea. Associated with the salt in the superficial beds, there are numerous, thin, horizontal layers of impure, dirty-white, friable, gypseous and calcareous tuffs. The gypseous beds are very remarkable, from abounding with, so as sometimes to be almost composed of, irregular concretions, from the size of an egg to that of a man's head, of very hard, compact, heavy gypsum, in the form of anhydrite. This gypsum contains some foreign...
Seite 466 - ... respects ; seeing that some fissile and almost homogeneous rocks show incipient mineralogical changes along the planes of their cleavage, and that other rocks with a fissile structure alternate with, and pass into varieties with a foliated structure, I cannot doubt that in most cases foliation and cleavage are parts of the same process : in cleavage there being only an incipient separation of the constituent minerals ; in foliation a much more complete separation and crystallisation.
Seite 152 - To descend into some of these valleys, it is necessary to go round twenty miles; and into others, the surveyors have only lately penetrated, and the colonists have not yet been able to drive in their cattle. But the most remarkable feature in their structure is, that although several miles wide at their heads, they generally contract towards their mouths to such a degree as to become impassable.
Seite 419 - Chili, or anywhere on the western coast of South America, naturally led Mr. Darwin to the conclusion that ' where the bed of the sea is either stationary or rising, circumstances are far less favourable than where the level is sinking to the accumulation of conchiferous strata of sufficient thickness and extension to resist the average vast amount of denudation.
Seite 151 - ... distance of several miles, he beholds another line, rising up to the same height with that on which he stands, and formed of the same horizontal strata of pale sandstone.
Seite 151 - It is not easy to conceive a more magnificent spectacle than is presented to a person walking on the summit-plains, when without any notice he arrives at the brink of one of these cliffs, which are so perpendicular that he can strike with a stone (as I have tried) the trees growing at a depth of 1,500 feet below him...
Seite 145 - Albemarle Island, which vents do not fall on the same line ; at the Canary Islands, eruptions have taken place in Teneriffe and Lanzarote ; and at the Azores, on the three parallel lines of Pico, St. Jorge and Terceira. Believing that a mountain-axis differs essentially from a volcano, only in plutonic rocks having been injected, instead of volcanic matter having been ejected, this appears to me an interesting circumstance ; for we may infer from it as probable, that in the elevation of a mountain-chain,...
Seite 80 - ... or to concretionary action, we should here have had the finest parallel zones of different composition and texture. In glaciers, the lines of porous ice and of minute crevices seem to be due to an incipient stretching, caused by the central parts of the frozen stream moving faster than the sides and bottom, which are retarded by friction. Hence, in glaciers of certain form, and towards the lower end of most glaciers, the zones become horizontal. May we venture to suppose that, in the feldspathic...
Seite 152 - ... river Grose joins the Nepean ; yet the valley of the Grose in its upper part, as I saw, forms a magnificent level basin...
Seite 352 - ... courses. The appearance and composition of the deposit, the manner in which it slopes up and round the primary ranges, the nature of the underlying marine beds, the estuary and sea-shells on the surface, the overlying sandstone beds at M. Hermoso, are all quite opposed to this view. Nor do I believe that there is a single instance of a skeleton of one of the extinct mammifers having been found in an upright position, as if it had been mired. The third theory, of the truth of which I cannot entertain...