Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 2 - By Alexander Von Humboldt And Aime Bonpland.
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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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