Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 1 - By Alexander Von Humboldt And Aime Bonpland.
- Page 319 of 407 - First - Home
We Continued To Descend From The Small Ravine Of Sanchorquiz To La
Cruz De La Guayra, A Cross Erected On An Open Spot, Six Hundred And
Thirty-Two Toises High, And Thence (Entering By The Custom-House
And The Quarter Of The Pastora) To The City Of Caracas.
On the
south side of the mountain of Avila, the gneiss presents several
geognostical phenomena worthy of the attention of travellers.
It is
traversed by veins of quartz, containing cannulated and often
articulated prisms of rutile titanite two or three lines in
diameter. In the fissures of the quartz we find, on breaking it,
very thin crystals, which crossing each other form a kind of
network. Sometimes the red schorl occurs only in dendritic crystals
of a bright red.* (* Especially below the Cross of La Guayra, at
594 toises of absolute elevation.) The gneiss of the valley of
Caracas is characterized by the red and green garnets it contains;
they however disappear when the rock passes into mica-slate. This
same phenomenon has been remarked by Von Buch in Sweden; but in the
temperate parts of Europe garnets are in general contained in
serpentine and mica-slates, not in gneiss. In the walls which
enclose the gardens of Caracas, constructed partly of fragments of
gneiss, we find garnets of a very fine red, a little transparent,
and very difficult to detach. The gneiss near the Cross of La
Guayra, half a league from Caracas, presented also vestiges of
azure copper-ore* (* Blue carbonate of copper.) disseminated in
veins of quartz, and small strata of plumbago (black lead), or
earthy carburetted iron. This last is found in pretty large masses,
and sometimes mingled with sparry iron-ore, in the ravine of
Tocume, to the west of the Silla.
Between the spring of Sanchorquiz and the Cross of La Guayra, as
well as still higher up, the gneiss contains considerable beds of
saccharoidal bluish-grey primitive limestone, coarse-grained,
containing mica, and traversed by veins of white calcareous spar.
The mica, with large folia, lies in the direction of the dip of the
strata. I found in the primitive limestone a great many
crystallized pyrites, and rhomboidal fragments of sparry iron-ore
of Isabella yellow. I endeavoured, but without success, to find
tremolite (Grammatite of Hauy. The primitive limestone above the
spring of Sanchorquiz, is directed, as the gneiss in that place,
hor. 5.2, and dips 45 degrees north; but the general direction of
the gneiss is, in the Cerro de Avila, hor. 3.4 with 60 degrees of
dip north-west. Exceptions merely local are observed in a small
space of ground near the Cross of La Guayra (hor. 6.2, dip 8
degrees north); and higher up, opposite the Quebrada of Tipe (hor.
12, dip 50 degrees west).), which in the Fichtelberg, in Franconia,
is common in the primitive limestone without dolomite. In Europe
beds of primitive limestone are generally observed in the
mica-slates; but we find also saccharoidal limestone in gneiss of
the most ancient formation, in Sweden near Upsala, in Saxony near
Burkersdorf, and in the Alps in the road over the Simplon.
Enter page number
PreviousNext
Page 319 of 407
Words from 165458 to 165982
of 211363
