These Insensible Passages From Primitive To Transition Strata By Clay-Slate, Which Becomes Carburetted At The Same Time That It Presents A Concordant Position With Mica-Slate And Gneiss, Have Also Been Observed Several Times In Europe By Celebrated Geologists.

The existence of an independent formation of primitive slate (urthonschiefer) may even be doubted, that is, of a formation which is not joined below by strata containing some vestiges of monocotyledonous plants.

The small thonschiefer bed of Malpasso (in the southern chain of the littoral Cordillera) is separated from mica-slate-gneiss by a co-ordinate formation of serpentine and diorite. It is divided into two shelves, of which the upper presents green steatitous slate mixed with amphibole, and the lower, dark-blue slate, extremely fissile, and traversed by numerous veins of quartz. I could discover no fragmentary stratum (grauwacke) nor kieselschiefer nor chiastolite. The kieselschiefer belongs in those countries to a limestone formation. I have seen fine specimens of the chiastolite (macle) which the Indians wore as amulets and which came from the Sierra Nevada de Merida. This substance is probably found in transition-slate, for MM. Rivero and Boussingault observed rocks of clay-slate at the height of 2120 toises, in the Paramo of Mucuchies, on going from Truxillo to Merida.* (* In Galicia, in Spain, I saw the thonschiefer containing chiastholite alternate with grauwacke; but the chiastolite unquestionably belongs also to rocks which all geologists have hitherto called primitive rocks, to mica-schists intercalated like layers in granite, and to an independent stratum of mica-slate.)

3. FORMATION OF SERPENTINE AND DIORITE (GREEN-STONE OF JUNCALITO.)

We have indicated above a layer of granitiferous serpentine inclosed in the gneiss of Buenavista, or perhaps superposed on that rock; we here find a real stratum of serpentine alternating with diorite, and extending from the ravine of Tucutunemo as far as Juncalito. Diorite forms the great mass of this stratum; it is of a dark green colour, granular, with small grains, and destitute of quartz; its mass is formed of small crystals of felspar intermixed with crystals of amphibole. This rock of diorite is covered at its surface, by the effect of decomposition, with a yellowish crust, like that of basalts and dolerites. Serpentine, of a dull olive-green and smooth fracture, mixed with bluish steatite and amphibole, presents, like almost all the co-ordinate formations of diorite and serpentine (in Silesia, at Fichtelgebirge, in the valley of Baigorry, in the Pyrenees, in the island of Cyprus and in the Copper Mountains of circumpolar America),* traces of copper. (* Franklin's Journey to the Polar Sea page 529.) Where the diorite, partly globular, approaches the green slate of Malpasso, real beds of green slate are found inclosed in diorite. The fine saussurite which we saw in the Upper Orinoco in the hands of the Indians, seems to indicate the existence of a soil of euphotide, superposed on gneiss-granite, or amphibolic slate, in the eastern part of the Sierra Parime.

4. GRANULAR AND MICACEOUS LIMESTONE OF THE MORROS OF SAN JUAN.

The Morros of San Juan rise like ruinous towers in a soil of diorite. They are formed of a cavernous greyish green limestone of crystalline texture, mixed with some spangles of mica, and are destitute of shells. We see in them masses of hardened clay, black, fissile, charged with iron, and covered with a crust, yellow from decomposition, like basalts and amphiboles. A compact limestone containing vestiges of shells adjoins this granular limestone of the Morros of San Juan which is hollow within. Probably on a further examination of the extraordinary strata between Villa de Cura and Ortiz, of which I had time only to collect some few specimens, many phenomena may be discovered analogous to those which Leopold von Buch has lately described in South Tyrol. M. Boussingault, in a memoir which he has recently addressed to me, calls the rock of the Morros a problematic calcariferous gneiss. This expression seems to prove that the plates of mica take in some parts a uniform direction, as in the greenish dolomite of Val Toccia.

5. FELSPATHIC SANDSTONE OF THE ORINOCO.

The gneiss-granite of the Sierra Parime is covered in some few places (between the Encaramada and the strait of Baraguan and in the island of Guachaco) in its western part with an olive-brown sandstone, containing grains of quartz and fragments of felspar, joined by an extremely compact clayey cement. This cement, where it abounds, has a conchoidal fracture and passes to jasper. It is crossed by small veins of brown iron-ore, which separate into very thin plates or scales. The presence of felspar seems to indicate that this small formation of sandstone (the sole secondary formation hitherto known in the Sierra Parime) belongs to red sandstone or coal.* (* Broken and intact crystals of feldspar are found in the todte liegende coal-sandstone of Thuringia. I observed in Mexico a very singular agglomerated felspar formation superposed upon (perhaps inclosed in) red sandstone, near Guanaxuato.) I hesitate to class it with the sandstone of the Llanos, the relative antiquity of which appears to me to be less satisfactorily verified.

6. FORMATION OF THE SANDSTONE OF THE LLANOS OF CALABOZO.

I arrange the various formations in the order which I fancied I could discern on the spot. The carburetted slate (thonschiefer) of the peninsula of Araya connects the primitive rocks of gneiss-granite and mica-slate-gneiss with the transition strata (blue and green slate, diorite, serpentine mixed with amphibole and granular greenish-grey limestone) of Malpasso, Tucutunemo and San Juan. On the south the sandstone of the Llanos rests on this transition strata; it is destitute of shells and composed, like the savannahs of Calabozo, of rounded fragments of quartz,* kieselschiefer and Lydian stone, cemented by a ferruginous olive-brown clay. (* In Germany sandstones which belong unquestionably to red sandstone contain also (near Weiderstadt, in Thuringia) nodules, and rounded fragments. I shall not cite the pudding-stone subordinate to the red sandstone of the Pyrenees because the age of that sandstone destitute of coal may be disputed.

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