The island of Quiriquina, in the Bay of Concepcion, is formed of various soft and often ferruginous sandstones, with bands of pebbles, and with the lower strata sometimes passing into a conglomerate resting on the underlying metamorphic schists.These beds include subordinate layers of greenish impure clay, soft micaceous and calcareous sandstones, and reddish friable earthy matter with white specks like decomposed crystals of feldspar; they include, also, hard concretions, fragments of shells, lignite, and silicified wood.In the upper part they pass into white, soft sediments and brecciolas, very like those described at Chiloe; as indeed is the whole formation.At Lirguen and other places on the eastern side of the bay, there are good sections of the lower sandstones, which are generally ferruginous, but which vary in character, and even pass into an argillaceous nature; they contain hard concretions, fragments of lignite, silicified wood, and pebbles (of the same rocks with the pebbles in the sandstones of Quiriquina), and they alternate with numerous, often very thin layers of imperfect coal, generally of little specific gravity.The main bed here is three feet thick; and only the coal of this one bed has a glossy fracture.Another irregular, curvilinear bed of brown, compact lignite, is remarkable for being included in a mass of coarse gravel.These imperfect coals, when placed in a heap, ignite spontaneously.The cliffs on this side of the bay, as well as on the island of Quiriquina, are capped with red friable earth, which, as stated in the Second Chapter, is of recent formation.The stratification in this neighbourhood is generally horizontal; but near Lirguen the beds dip N.W.at an angle of 23 degrees;near Concepcion they are also inclined: at the northern end of Quiriquina they have been tilted at an angle of 30 degrees, and at the southern end at angles varying from 15 degrees to 40 degrees: these dislocations must have taken place under the sea.
A collection of shells, from the island of Quiriquina, has been described by M.d'Orbigny: they are all extinct, and from their generic character, M.
d'Orbigny inferred that they were of tertiary origin: they consist of:--1.Scalaria Chilensis, d'Orbigny, "Voyage, Part Pal."2.Natica Araucana, d'Orbigny, "Voyage, Part Pal."3.Natica australis, d'Orbigny, "Voyage, Part Pal."4.Fusus difficilis, d'Orbigny, "Voyage, Part Pal."5.Pyrula longirostra, d'Orbigny, "Voyage, Part Pal."6.Pleurotoma Araucana, d'Orbigny, "Voyage, Part Pal."7.Cardium auca, d'Orbigny, "Voyage, Part Pal."8.Cardium acuticostatum, d'Orbigny, "Voyage, Part Pal."9.Venus auca, d'Orbigny, "Voyage, Part Pal."10.Mactra cecileana, d'Orbigny, "Voyage, Part Pal."11.Mactra Araucana, d'Orbigny, "Voyage, Part Pal."12.Arca Araucana, d'Orbigny, "Voyage, Part Pal."13.Nucula Largillierti, d'Orbigny, "Voyage, Part Pal."14.Trigonia Hanetiana, d'Orbigny, "Voyage, Part Pal."During a second visit of the "Beagle" to Concepcion, Mr.Kent collected for me some silicified wood and shells out of the concretions in the sandstone from Tome, situated a short distance north of Lirguen.They consist of:--1.Natica australis, d'Orbigny, "Voyage, Part Pal."2.Mactra Araucana, d'Orbigny, "Voyage, Part Pal."3.Trigonia Hanetiana, d'Orbigny, "Voyage, Part Pal."4.Pecten, fragments of, probably two species, but too imperfect for description.
5.Baculites vagina, E.Forbes.
6.Nautilus d'Orbignyanus, E.Forbes.
Besides these shells, Captain Belcher found here an Ammonite, nearly three feet in diameter, and so heavy that he could not bring it away; fragments are deposited at Haslar Hospital: he also found the silicified vertebrae of some very large animal.("Zoology of Captain Beechey's Voyage" page 163.)>From the identity in mineralogical nature of the rocks, and from Captain Belcher's minute description of the coast between Lirguen and Tome, the fossiliferous concretions at this latter place certainly belong to the same formation with the beds examined by myself at Lirguen; and these again are undoubtedly the same with the strata of Quiriquina; moreover; the three first of the shells from Tome, though associated in the same concretions with the Baculite, are identical with the species from Quiriquina.Hence all the sandstone and lignitiferous beds in this neighbourhood certainly belong to the same formation.Although the generic character of the Quiriquina fossils naturally led M.d'Orbigny to conceive that they were of tertiary origin, yet as we now find them associated with the Baculites vagina and with an Ammonite, we must, in the opinion of M.d'Orbigny, and if we are guided by the analogy of the northern hemisphere, rank them in the Cretaceous system.Moreover, the Baculites vagina, which is in a tolerable state of preservation, appears to Professor E.Forbes certainly to be identical with a species, so named by him, from Pondicherry in India;where it is associated with numerous decidedly cretaceous species, which approach most nearly to Lower Greensand or Neocomian forms: this fact, considering the vast distance between Chile and India, is truly surprising.
Again, the Nautilus d'Orbignyanus, as far as its imperfect state allows of comparison, resembles, as I am informed by Professor Forbes, both in its general form and in that of its chambers, two species from the Upper Greensand.It may be added that every one of the above-named genera from Quiriquina, which have an apparently tertiary character, are found in the Pondicherry strata.There are, however, some difficulties on this view of the formations at Concepcion being cretaceous, which I shall afterwards allude to; and I will here only state that the Cardium auca is found also at Coquimbo, the beds at which place, there can be no doubt, are tertiary.