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.
Relations between ancient orifices of eruption and subsequent axes of injection.
Iquique, Peru, fossils of, salt-deposits.
Metalliferous veins.
Summary on the porphyritic conglomerate and gypseous formations.
Great subsidence with partial elevations during the cretaceo-oolitic period.
On the elevation and structure of the Cordillera.
Recapitulation on the tertiary series.
Relation between movements of subsidence and volcanic action.
Pampean formation.
Recent elevatory movements.
Long-continued volcanic action in the Cordillera.
Conclusion.
VALPARAISO TO COQUIMBO.
I have already described the general nature of the rocks in the low country north of Valparaiso, consisting of granites, syenites, greenstones, and altered feldspathic clay-slate.Near Coquimbo there is much hornblendic rock and various dusky-coloured porphyries.I will describe only one section in this district, namely, from near Illapel in a N.E.line to the mines of Los Hornos, and thence in a north by east direction to Combarbala, at the foot of the main Cordillera.
Near Illapel, after passing for some distance over granite, andesite, and andesitic porphyry, we come to a greenish stratified feldspathic rock, which I believe is altered clay-slate, conformably capped by porphyries and porphyritic conglomerate of great thickness, dipping at an average angle of 20 degrees to N.E.by N.The uppermost beds consist of conglomerates and sandstone only a little metamorphosed, and conformably covered by a gypseous formation of very great thickness, but much denuded.This gypseous formation, where first met with, lies in a broad valley or basin, a little southward of the mines of Los Hornos: the lower half alone contains gypsum, not in great masses as in the Cordillera, but in innumerable thin layers, seldom more than an inch or two in thickness.The gypsum is either opaque or transparent, and is associated with carbonate of lime.The layers alternate with numerous varying ones of a calcareous clay-shale (with strong aluminous odour, adhering to the tongue, easily fusible into a pale green glass), more or less indurated, either earthy and cream-coloured, or greenish and hard.The more indurated varieties have a compact, homogeneous, almost crystalline fracture, and contain granules of crystallised oxide of iron.Some of the varieties almost resemble honestones.There is also a little black, hardly fusible, siliceo-calcareous clay-slate, like some of the varieties alternating with gypsum on the Peuquenes range.
The upper half of this gypseous formation is mainly formed of the same calcareous clay-shale rock, but without any gypsum, and varying extremely in nature: it passes from a soft, coarse, earthy, ferruginous state, including particles of quartz, into compact claystones with crystallised oxide of iron,--into porcellanic layers, alternating with seams of calcareous matter,--and into green porcelain-jasper, excessively hard, but easily fusible.Strata of this nature alternate with much black and brown siliceo-calcareous slate, remarkable from the wonderful number of huge embedded logs of silicified wood.This wood, according to Mr.R.Brown, is (judging from several specimens) all coniferous.Some of the layers of the black siliceous slate contained irregular angular fragments of imperfect pitchstone, which I believe, as in the Uspallata range, has originated in a metamorphic process.There was one bed of a marly tufaceous nature, and of little specific gravity.Veins of agate and calcareous spar are numerous.
The whole of this gypseous formation, especially the upper half, has been injected, metamorphosed, and locally contorted by numerous hillocks of intrusive porphyries crowded together in an extraordinary manner.These hillocks consist of purple claystone and of various other porphyries, and of much white feldspathic greenstone passing into andesite; this latter variety included in one case crystals of orthitic and albitic feldspar touching each other, and others of hornblende, chlorite, and epidote.The strata surrounding these intrusive hillocks at the mines of Los Hornos, are intersected by many veins of copper-pyrites, associated with much micaceous iron-ore, and by some of gold: in the neighbourhood of these veins the rocks are blackened and much altered.The gypsum near the intrusive masses is always opaque.One of these hillocks of porphyry was capped by some stratified porphyritic conglomerate, which must have been brought up from below, through the whole immense thickness of the overlying gypseous formation.The lower beds of the gypseous formation resemble the corresponding and probably contemporaneous strata of the main Cordillera;whilst the upper beds in several respects resemble those of the Uspallata chain, and possibly may be contemporaneous with them; for I have endeavoured to show that the Uspallata beds were accumulated subsequently to the gypseous or Neocomian formations of the Cordillera.
