These Transitions And Alternating Superpositions Take Place No Doubt In Formations Of Granite-Gneiss And Gneiss-Mica-Slate; But Because These Phenomena Are Observed In One Region, It Does Not Follow That In Other Regions We May Not Find Very Distinct Circumscribed Formations Of Granite, Gneiss, And Mica-Slate.

The same considerations may be applied to the formations of serpentine, which are sometimes isolated, and sometimes belong to the eurite, mica-slate, and grunstein.) Beyond the town of Villa de Cura and the Cerro de Chacao the aspect of the country presents greater geognostic variety.

There are still eight leagues of declivity from the table-land of Cura to the entry of the Llanos; and on the southern slope of the mountains of the coast, four different formations of rock cover the gneiss. We shall first give the description of the different strata, without grouping them systematically.

On the south of the Cerro de Chacao, between the ravine of Tucutunemo and Piedras Negras, the gneiss is concealed beneath a formation of serpentine, of which the composition varies in the different superimposed strata. Sometimes it is very pure, very homogeneous, of a dusky olive-green, and of a conchoidal fracture: sometimes it is veined, mixed with bluish steatite, of an unequal fracture, and containing spangles of mica. In both these states I could not discover in it either garnets, hornblende, or diallage. Advancing farther to the south (and we always passed over this ground in that direction) the green of the serpentine grows deeper, and feldspar and hornblende are recognised in it: it is difficult to determine whether it passes into diabasis or alternates with it. There is, however, no doubt of its containing veins of copper-ore.* (* One of these veins, on which two shafts have been sunk, was directed hor. 2.1, and dipped 80 degrees east. The strata of the serpentine, where it is stratified with some regularity, run hor. 8, and dip almost perpendicularly. I found malachite disseminated in this serpentine, where it passes into grunstein.) At the foot of this mountain two fine springs gush out from the serpentine. Near the village of San Juan, the granular diabasis appears alone uncovered, and takes a greenish black hue. The feldspar intimately mixed with the mass, may be separated into distinct crystals. The mica is very rare, and there is no quartz. The mass assumes at the surface a yellowish crust like dolerite and basalt.

In the midst of this tract of trap-formation, the Morros of San Juan rise like two castles in ruins. They appear linked to the mornes of St. Sebastian, and to La Galera which bounds the Llanos like a rocky wall. The Morros of San Juan are formed of limestone of a crystalline texture; sometimes very compact, sometimes spongy, of a greenish-grey, shining, composed of small grains, and mixed with scattered spangles of mica. This limestone yields a strong effervescence with acids. I could not find in it any vestige of organized bodies.

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