It Contains In Subordinate Strata, Masses Of Hardened Clay Of A Blackish Blue, And Carburetted.

These masses are fissile, very heavy, and loaded with iron; their streak is whitish, and they produce no effervescence with acids.

They assume at their surface, by their decomposition in the air, a yellow colour. We seem to recognize in these argillaceous strata a tendency either to the transition-slates, or to the kieselschiefer (schistose jasper), which everywhere characterise the black transition-limestones. When in fragments, they might be taken at first sight for basalt or hornblende.* (* I had an opportunity of examining again, with the greatest care, the rocks of San Juan, of Chacao, of Parapara, and of Calabozo, during my stay at Mexico, where, conjointly with M. del Rio, one of the most distinguished pupils of the school of Freyberg, I formed a geognostical collection for the Colegio de Mineria of New Spain.) Another white limestone, compact, and containing some fragments of shells, backs the Morros de San Juan. I could not see the line of junction of these two limestones, or that of the calcareous formation and the diabasis.

The transverse valley which descends from Piedras Negras and the village of San Juan, towards Parapara and the Llanos, is filled with trap-rocks, displaying close affinity with the formation of green slates, which they cover. Sometimes we seem to see serpentine, sometimes grunstein, and sometimes dolerite and basalt. The arrangement of these problematical masses is not less extraordinary. Between San Juan, Malpaso, and Piedras Azules, they form strata parallel to each other; and dipping regularly northward at an angle of 40 or 50 degrees, they cover even the green slates in concordant stratification. Lower down, towards Parapara and Ortiz, where the amygdaloids and phonolites are connected with the grunstein, everything assumes a basaltic aspect. Balls of grunstein heaped one upon another, form those rounded cones, which are found so frequently in the Mittelgebirge in Bohemia, near Bilin, the country of phonolites. The following is the result of my partial observations.

The grunstein, which at first alternated with strata of serpentine, or was connected with that rock by insensible transitions, is seen alone, sometimes in strata considerably inclined, and sometimes in balls with concentric strata, imbedded in strata of the same substance. It lies, near Malpaso, on green slates, steatitic, mingled with hornblende, destitute of mica and grains of quartz, dipping, like the grunsteins, 45 degrees toward the north, and directed, like them, 75 degrees north-west.

A great sterility prevails where these green slates predominate, no doubt on account of the magnesia they contain, which (as is proved by the magnesian-limestone of England*) is very hurtful to vegetation. (* Magnesian limestone is of a straw-yellow colour, and contains madrepores: it lies beneath red marl, or muriatiferous red sandstone.) The dip of the green slates continues the same; but by degrees the direction of their strata becomes parallel to the general direction of the primitive rocks of the chain of the coast.

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