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M. Domeyko has sent to France a collection of fossils, which, I presume, from the description given, must have come from the neighbourhood of Arqueros; they consist of: Pecten Dufreynoyi, d'Orbigny, "Voyage" Part Pal. Ostrea hemispherica, d'Orbigny, "Voyage" Part Pal. Turritella Andii, d'Orbigny, "Voyage" Part Pal. Hippurites Chilensis, d'Orbigny, "Voyage" Part Pal.
The upper beds in this section probably correspond with parts of the great gypseous formation; and the lower beds of red sandstone conglomerate and fossiliferous limestone no doubt are the equivalents of the Hippurite stratum, seen in descending from Arqueros to Pluclaro, which there lies conformably upon the porphyritic conglomerate formation. The fossils found in the third bed, consist of:
Collected by M. Domeyko from the same locality, apparently near Arqueros: Terebratula aenigma and Terebratula ignaciana, in same block of limestone: Pecten Dufreynoyi. Ostrea hemispherica. Hippurites Chilensis. Turritella Andii. Nautilus Domeykus. In a collection from the Cordillera, given me: the specimens all in the same condition: Pecten Dufreynoyi. Turritella Andii. Terebratula ignaciana.
Sixthly, a thick mass, obscurely stratified, of a red sedimentary stone or sandstone, full of crystalline calcareous matter, imperfect crystals of oxide of iron, and I believe of feldspar, and therefore closely resembling some of the highly metamorphosed beds at Arqueros: this bed was capped by, and appeared to pass in its upper part into, rocks similarly coloured, containing calcareous matter, and abounding with minute crystals, mostly elongated and glassy, of reddish albite.
I will reiterate the evidence on the association of these several shells in the several localities. In the same bed, Rio Claro: Pecten Dufreynoyi. Ostrea hemispherica. Terebratula aenigma. Spirifer linguiferoides. Same bed, near Arqueros: Hippurites Chilensis. Gryphaea orientalis.
In descending from the Arqueros district, I crossed on the northern border of the valley, strata inclined eastward from the Pluclaro axis: on the porphyritic conglomerate there rested a mass, some hundred feet thick, of brown argillaceous limestone, in parts crystalline, and in parts almost composed of Hippurites Chilensis, d'Orbigny; above this came a black calcareous shale, and on it a red conglomerate.
Silver, in the form of a chloride, sulphuret, or an amalgam, or in its native state, and associated with lead and other metals, and at Arqueros with pure native copper, occurs chiefly in the upper great gypseous or cretaceo-oolitic formation which forms probably the richest mass in Chile. Hence comes Molina's remark, that silver is born in the cold and solitary deserts of the Upper Cordillera.
I believe that this anticlinal axis can be traced northwards, into the district of Arqueros, where a conspicuous hill called Cerro Blanco, formed of a harsh, cream-coloured euritic rock, including a few crystals of reddish feldspar, and associated with some purplish claystone porphyry, seems to fall on a line of elevation.
May 21st. I set out in company with Don Jose Edwards to the silver-mine of Arqueros, and thence up the valley of Coquimbo. Passing through a mountainous country, we reached by nightfall the mines belonging to Mr. Edwards. I enjoyed my night's rest here from a reason which will not be fully appreciated in England, namely, the absence of fleas!
Section from Illapel to Combarbala; gypseous formation with silicified wood. Panuncillo. Coquimbo; mines of Arqueros; section up valley; fossils. Guasco, fossils of. Copiapo, section up valley; Las Amolanas, silicified wood. Conglomerates, nature of former land, fossils, thickness of strata, great subsidence. Valley of Despoblado, fossils, tufaceous deposit, complicated dislocations of.