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Few communications have excited more interest in the Society than the letters on South America addressed by Mr. Charles Darwin to Professor Henslow. Mr. Darwin has devoted four years, from 1832 to 1835 inclusive, to the investigation of the natural history and geology of South America. From the position of the tertiary deposits which exist on both sides of the southern Andes, he concludes that the primary chain must have had a great elevation anterior to the tertiary period. A transverse section from Rio Santa Cruz to the base of the Cordilleras, and another on the Rio Negro exhibit the structure of what Mr. Darwin calls the great southern tertiary formations of Patagonia, which may be separated into groups of distinct periods analogous to those already established in Europe. The lowest group is of great extent and thickness, and in one instance was observed to alternate with a bed of ancient lava, which seemed to mark the commencement of the eruptions from the craters of the principal chain of the Andes. Among the shells and corals, even of this lowest deposit, are some which are supposed to belong to species now living in the neighbouring Pacific. Overlying this is a stratum of rolled porphyry pebbles, which the author traced for 700 miles. Scattered over the whole, and at various heights above the sea, from 1300 feet downwards, are recent shells of littoral species of the neighbouring coast, so that every part of the surface seems once to have been a shore, and Mr. Darwin supposes that an upheaval to the amount of 1300 feet has been owing to a succession of small elevations, like those experienced in modern times in Chili.
The principal section described is one transverse to the Andes, extending from Valparaiso to Mendoza. 'The Cordillera consists here of two separate and parallel chains, the western being composed of stratified sedimentary rocks resting on granite. The strata are violently dislocated and contorted along parallel north and south lines, and become crystalline as they approach the gra-
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nite. Some of the slates and limestones, probably referable to the transition period, contain organic remains at an elevation of 13,000 -feet above the sea. In the eastern chain are sandstones and conglomerates, and associated felspathic rocks regularly bedded, and more recent than the rocks of the western chain, being partly made up of their debris. After much investigation Mr. Darwin convinced himself that these were of the same age with certain tertiary deposits of Patagonia, Chiloe, and Conception, resembling them in mineral character and in the lignite and fossil wood which they contain. In one escarpment is seen a sandstone of this system in which there is a wood of petrified trees in a vertical position, some of the trees being perfectly silicified and of dicotyledonous wood, others consisting of snow-white columns of coarsely crystallized carbonate of lime. They appear to have formed a clump of trees which had grown on lava and was then submerged, so that layers of fine sandstone were quietly deposited between the trunks. The enveloping sandstone rests on lava, and is again covered by a bed of black augitic lava about 1000 feet thick. Over this there are at least five other grand alternations of similar rocks and aqueous deposits, amounting in thickness to several thousand feet. The same sedimentary strata, or the continuation of them, are not only altered by granite, but are traversed by dikes of granite proceeding from the mass, and also by numerous metallic veins of iron, copper, arsenic, silver, and gold, all of which can be traced to the underlying granite. A gold mine has been worked close to the clump of silicified trees.

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… Mr. Darwin has also observed in different parts of Patagonia and Chili beds of recent shells at various heights above

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the sea, and among them mussels which retained their blue colour, and emit a strong animal odour when thrown into the fire.

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I have before adverted to the petrified forest described by Mr. Darwin, in Chili, where the trees have grown on a bed of lava, and have then been covered by sand and sedimentary and volcanic matter 2000 feet thick. These facts seem to prove that the region of the Andes, instead of having been raised up suddenly and at once, a few thousand years before our time, as some have conjectured, has undergone, even since the commencement of the tertiary period, vast movements of depression as well as of elevation.