This formation is intersected by numerous metalliferous veins, running, though irregularly, N.W.and S.E., and generally at right angles to the many dikes.The veins consist of native silver, of muriate of silver, an amalgam of silver, cobalt, antimony, and arsenic, generally embedded in sulphate of barytes.(See the Report on M.Domeyko's account of those mines, in the "Comptes Rendus" tome 14 page 560.) I was assured by Mr.
Lambert, that native copper without a trace of silver has been found in the same vein with native silver without a trace of copper.At the mines of Aristeas, the silver veins are said to be unproductive as soon as they pass into the green strata, whereas at S.Rosa, only two or three miles distant, the reverse happens; and at the time of my visit, the miners were working through a red stratum, in the hope of the vein becoming productive in the underlying green sedimentary mass.I have a specimen of one of these green rocks, with the usual granules of white calcareous spar and red oxide of iron, abounding with disseminated particles of glittering native and muriate of silver, yet taken at the distance of one yard from any vein,--a circumstance, as I was assured, of very rare occurrence.
SECTION EASTWARD, UP THE VALLEY OF COQUIMBO.
After passing for a few miles over the coast granitic series, we come to the porphyritic conglomerate, with its usual characters, and with some of the beds distinctly displaying their mechanical origin.The strata, where first met with, are, as before stated, only slightly inclined; but near the Hacienda of Pluclaro, we come to an anticlinal axis, with the beds much dislocated and shifted by a great fault, of which not a trace is externally seen in the outline of the hill.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.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.In the brown limestone, with the Hippurites, there was an impression of a Pecten and a coral, and great numbers of a large Gryphaea, very like, and, according to Professor E.Forbes, probably identical with G.Orientalis, Forbes MS.,--a cretaceous species (probably upper greensand) from Verdachellum, in Southern India.
These fossils seem to occupy nearly the same position with those at the Puente del Inca,--namely, at the top of the porphyritic conglomerate, and at the base of the gypseous formation.
A little above the Hacienda of Pluclaro, I made a detour on the northern side of the valley, to examine the superincumbent gypseous strata, which Iestimated at 6,000 feet in thickness.The uppermost beds of the porphyritic conglomerate, on which the gypseous strata conformably rest, are variously coloured, with one very singular and beautiful stratum composed of purple pebbles of various kinds of porphyry, embedded in white calcareous spar, including cavities lined with bright-green crystallised epidote.The whole pile of strata belonging to both formations is inclined, apparently from the above-mentioned axis of Pluclaro, at an angle of between 20 and 30degrees to the east.I will here give a section of the principal beds met with in crossing the entire thickness of the gypseous strata.
Firstly: above the porphyritic conglomerate formation, there is a fine-grained, red, crystalline sandstone.
Secondly: a thick mass of smooth-grained, calcareo-aluminous, shaly rock, often marked with dendritic manganese, and having, where most compact, the external appearance of honestone.It is easily fusible.I shall for the future, for convenience' sake, call this variety pseudo-honestone.Some of the varieties are quite black when freshly broken, but all weather into a yellowish-ash coloured, soft, earthy substance, precisely as is the case with the compact shaly rocks of the Peuquenes range.This stratum is of the same general nature with many of the beds near Los Hornos in the Illapel section.In this second bed, or in the underlying red sandstone (for the surface was partially concealed by detritus), there was a thick mass of gypsum, having the same mineralogical characters with the great beds described in our sections across the Cordillera.
Thirdly: a thick stratum of fine-grained, red, sedimentary matter, easily fusible into a white glass, like the basis of claystone porphyry; but in parts jaspery, in parts brecciated, and including crystalline specks of carbonate of lime.In some of the jaspery layers, and in some of the black siliceous slaty bands, there were irregular seams of imperfect pitchstone, undoubtedly of metamorphic origin, and other seams of brown, crystalline limestone.Here, also, were masses, externally resembling ill-preserved silicified wood.
Fourthly and fifthly: calcareous pseudo-honestone; and a thick stratum concealed by detritus.
