GENERAL PARTITION OF GROUND.
DIRECTION AND INCLINATION OF THE STRATA.
RELATIVE HEIGHT OF THE FORMATIONS ABOVE THE LEVEL OF THE OCEAN.
In the preceding section we have examined the inequalities of the
surface of the soil, that is to say, the general structure of the
mountains and the form of the basins rising between those variously
grouped mountains. These mountains are sometimes longitudinal, running
in narrow bands or chains, similar to the veins that preserve their
directions at great distances, as the Andes, the littoral chain of
Venezuela, the Serra do Mar of Brazil, and the Alleghenies of the
United States. Sometimes they are in masses with irregular forms, in
which upheavings seem to have taken place as on a labyrinth of
crevices or a heap of veins, as for example in the Sierra Parime and
the Serra dos Vertentes. These modes of formation are linked with a
geognostic hypothesis, which has at least the recommendation of being
founded on facts observed in remote times, and which strongly
characterize the chains and groups of mountains. Considerations on the
aspect of a country are independent of those which indicate the nature
of the soil, the heterogeneity of matter, the superposition of rocks
and the direction and inclination of strata.
In taking a general view of the geological constitution of a chain of
mountains, we may distinguish five elements of direction too often
confounded in works of geognosy and physical geography. These elements
are:--
1. The longitudinal axis of the whole chain.
2. The line that divides the waters (divortia aquarum).
3. The line of ridges or elevation passing along the maxima of height.
4. The line that separates two contiguous formations into horizontal
sections.
5. The line that follows the fissures of stratification.
This distinction is the more necessary, there existing probably no
chain on the globe that furnishes a perfect parallelism of all these
directing lines. In the Pyrenees, for instance, 1, 2, 3, do not
coincide, but 4 and 5 (that is, the different formations which come to
light successively, and the direction of the strata) are obviously
parallel to 1, or to the direction of the whole chain. We find so
often in the most distant parts of the globe, a perfect parallelism
between 1 and 5, that it may be supposed that the causes which
determine the direction of the axis (the angle under which that axis
cuts the meridian) are generally linked with causes that determine the
direction and inclination of the strata. This direction of the strata
is independent of the line of the formations, or their visible limits
at the surface of the soil; the lines 4 and 5 sometimes cross each
other, even when one of them coincides with 1, or with the direction
of the longitudinal axis of the whole chain. The RELIEF of a country
cannot be precisely explained on a map, nor can the most erroneous
opinions on the locality and superposition of the strata be avoided,
if we do not apprehend with clearness the relation of the directing
lines just mentioned.
In that part of South America to which this memoir principally
relates, and which is bounded by the Amazon on the south, and on the
west by the meridian of the Snowy Mountains (Sierra Nevada) of Merida,
the different bands or zones of formations (4) are sensibly parallel
with the longitudinal axis (1) of the chains of mountains, basins or
interposed plains. It may be said in general that the granitic zone
(including under that denomination the rocks of granite, gneiss and
mica-slate) follows the direction of the Cordillera of the shore of
Venezuela, and belongs exclusively to that Cordillera and the group of
the Parime mountains; since it nowhere pierces the secondary and
tertiary strata in the Llanos or basin of the Lower Orinoco. Thence it
results that the same formations do not constitute the region of
plains and that of mountains.
If we may be allowed to judge of the structure of the whole Sierra
Parime, from the part which I examined in 6 degrees of longitude, and
4 degrees of latitude, we may believe it to be entirely composed of
gneiss-granite; I saw some beds of greenstone and amphibolic slate,
but neither mica-slate, clay-slate, nor banks of green limestone,
although many phenomena render the presence of mica-slate probable on
the east of the Maypures and in the chain of Pacaraina. The geological
formation of the Parime group is consequently still more simple than
that of the Brazilian group, in which granites, gneiss and mica-slate
are covered with thonschiefer, chloritic quartz (Itacolumite),
grauwacke and transition-limestone; but those two groups exhibit in
common the absence of a real system of secondary rocks; we find in
both only some fragments of sandstone or silicious conglomerate. In
the littoral Cordillera of Venezuela the granitic formations
predominate; but they are wanting towards the east, and especially in
the southern chain, where we observe (in the missions of Caripe and
around the gulf of Cariaco) a great accumulation of secondary and
tertiary calcareous rocks. From the point where the littoral
Cordillera is linked with the Andes of New Grenada (longitude 71 1/2
degrees) we observe first the granitic mountains of Aroa and San
Felipe, between the rivers Yaracui and Tocuyo; these granitic
formations extend on the east of the two coasts of the basin of the
Valleys of Aragua, in the northern chain, as far as Cape Codera; and
in the southern as far as the mountains (altas savanas) of Ocumare.
After the remarkable interruption of the littoral Cordillera in the
province of Barcelona, granitic rocks begin to appear in the island of
Marguerita and in the isthmus of Araya, and continue, perhaps, towards
the Boca del Drago; but on the east of the meridian of Cape Codera the
northern chain only is granitic (of micaceous slate); the southern
chain is entirely composed of secondary limestone and sandstone.
If, in the granitic series, where a very complex formation, we would
distinguish mineralogically between the rocks of granite, gneiss, and
mica-slate, it must be borne in mind that coarse-grained granite, not
passing to gneiss, is very rare in this country. It belongs peculiarly
to the mountains that bound the basin of the lake of Valencia towards
the north; for in the islands of that lake, in the mountains near the
Villa de Cura, and in the whole northern chain, between the meridian
of Vittoria and Cape Codera, gneiss predominates, sometimes
alternating with granite, or passing to mica-slate. Mica-slate is the
most frequent rock in the peninsula of Araya and the group of Macanao,
which forms the western part of the island of Marguerita. On the west
of Maniquarez the mica-slate of the peninsula of Araya loses by
degrees its semi-metallic lustre; it is charged with carbon, and
becomes a clay-slate (thonschiefer) even an ampelite (alaunschiefer).
Beds of granular limestone are most common in the primitive northern
chain; and it is somewhat remarkable that they are found in gneiss,
and not in mica-slate.
We find at the back of this granitic, or rather mica-slate-gneiss soil
of the southern chain, on the south of the Villa de Cura, a transition
stratum, composed of greenstone, amphibolic serpentine, micaceous
limestone, and green and carburetted slate. The most southern limit of
this district is marked by volcanic rocks. Between Parapara, Ortiz and
the Cerro de Flores (latitude 9 degrees 28 minutes to 9 degrees 34
minutes; longitude 70 degrees 2 minutes to 70 degrees 15 minutes)
phonolites and amygdaloids are found on the very border of the basin
of the Llanos, that vast inland sea which once filled the whole space
between the Cordilleras of Venezuela and Parime. According to the
observations of Major Long and Dr. James, trap-formations (bulleuses
dolerites and amygdaloids with pyroxene) also border the plains or
basin of the Mississippi, towards the west, at the declivity of the
Rocky Mountains. The ancient pyrogenic rocks which I found near
Parapara where they rise in mounds with rounded summits, are the more
remarkable as no others have hitherto been discovered in the whole
eastern part of South America. The close connection observed in the
strata of Parapara, between greenstone, amphibolic serpentine, and
amygdaloids containing crystals of pyroxene; the form of the Morros of
San Juan, which rise like cylinders above the table-land; the granular
texture of their limestone, surrounded by trap rocks, are objects
worthy the attention of the geologist who has studied in the southern
Tyrol the effects produced by the contact of poroxenic porphyries.* (*
Leopold von Buch. Tableau geologique du Tyrol page 17. M. Boussingault
states that these singular Morros de San Juan, which furnish a
limestone with crystalline grains, and thermal springs, are hollow,
and contain immense grottos filled with stalactites, which appear to
have been anciently inhabited by the natives.)
The calcareous soil of the littoral Cordillera prevails most on the
east of Cape Unare, in the southern chain; it extends to the gulf of
Paria, opposite the island of Trinidad, where we find gypsum of Guire,
containing sulphur. I have been informed that in the northern chain
also, in the Montana de Paria, and near Carupana, secondary calcareous
formations are found, and that they only begin to show themselves on
the east of the ridge of rock called the Cerro de Meapire, which joins
the calcareous group of Guacharo to the mica-slate group of the
peninsula of Araya; but I have not had an opportunity of ascertaining
the accuracy of this information. The calcareous stratum of the
southern chain is composed of two formations which appear to be very
distinct the one from the other: namely limestone of Cumanacoa and
that of Caripe. When I was on the spot the former appeared to me to
have some analogy with zechstein, or Alpine limestone; the latter with
Jura limestone; I even thought that the granular gypsum of Guire might
be that which belongs in Europe to zechstein, or is placed between
zechstein and variegated sandstone. Strata of quartzose sandstone,
alternating with slaty clay, cover the limestone of Cumanacoa, Cerro
del Imposible, Turimiquiri, Guarda de San Agustin, and the Jura
limestone in the province of Barcelona (Aguas Calientes). According to
their position these sandstones may be considered as belonging to the
formation of green sandstone, or sandstone with lignites below chalk.
But if, as I thought I observed at Cocollar, sandstone forms strata in
the Alpine limestone before it is superposed, it appears doubtful
whether the sandstone of the Imposible, and of Aguas Calientes,
constitute one series. Muriatiferous clay (with petroleum and lamellar
gypsum) covers the western part of the peninsula of Araya, opposite to
the town of Cumana, and in the centre of the island of Marguerita.
This clay appears to lie immediately over the mica-slate, and under
the calcareous breccia of the tertiary strata. I cannot decide whether
Araya, which is rich in disseminated muriate of soda, belongs to the
sandstone formation of the Imposible, which from its position may be
compared to variegated sandstone (red marl).
There is no doubt that fragments of tertiary strata surround the
castle and town of Cumana (Castillo de San Antonio) and they also
appear at the south-western extremity of the peninsula of Araya (Cerro
de la Vela et del Barigon); at the ridge of the Cerro de Meapire, near
Cariaco; at Cabo Blanco, on the west of La Guayra, and on the shore of
Porto Cabello; they are consequently found at the foot of the two
slopes of the northern chain of the Cordillera of Venezuela. This
tertiary stratum is composed of alternate beds of calcareous
conglomerate, compact limestone, marl, and clay, containing selenite
and lamellar gypsum. The whole system (of very recent beds) appears to
me to constitute but one formation, which is found at the Cerro de la
Popa, near Carthagena, and in the islands of Guadaloupe and Martinico.
Such is the geological distribution of strata in the mountainous part
of Venezuela, in the group of the Parime and in the littoral
Cordillera. We have now to characterize the formations of the Llanos
(or of the basin of the Lower Orinoco and the Apure); but it is not
easy to determine the order of their superposition, because in this
region ravines or beds of torrents and deep wells dug by the hands of
man are entirely wanting. The formations of the Llanos are, first, a
sandstone or conglomerate, with rounded fragments of quartz, Lydian
stone, and kieselschiefer, united by a ferruginous clayey cement,
extremely tenacious, olive-brown, sometimes of a vivid red; second, a
compact limestone (between Tisnao and Calabozo) which, by its smooth
fracture and lithographic aspect, approaches the Jura limestone:
third, alternate strata of marl and lamellar gypsum (Mesa de San
Diego, Ortiz, Cachipo). These three formations appeared to me to
succeed each other in the order I have just described, the sandstone
inclining in a concave position, northward, on the transition-slates
of Malpasso, and southward, on the gneiss-granite of Parime. As the
gypsum often immediately covers the sandstone of Calabozo, which
appeared to me, on the spot, to be identical with our red sandstone, I
am uncertain of the age of its formation. The secondary rocks of the
Llanos of Cumana, Barcelona and Caracas occupy a space of more than
5000 square leagues. Their continuity is the more remarkable, as they
appear to have no existence, at least on the east of the meridian of
Porto Cabello (70 degrees 37 minutes) in the whole basin of the Amazon
not covered by granitic sands. The causes which have favoured the
accumulation of calcareous matter in the eastern region of the coast
chain, in the Llanos of Venezuela (from 10 1/2 to 8 degrees north),
cannot have operated nearer the equator, in the group of the mountains
of the Parime and in the plains of the Rio Negro and the Amazon
(latitude 1 degree north to 1 degree south). The latter plains,
however, furnish some ledges of fragmentary rocks on the south-west of
San Fernando de Atabapo, as well as on the south-east, in the lower
part of the Rio Negro and the Rio Branco. I saw in the plains of Jaen
de Bracamoros a sandstone which alternates with ledges of sand and
conglomerate nodules of porphyry and Lydian stone. MM. Spix and
Martius affirm that the banks of the Rio Negro on the south of the
equator are composed of variegated sandstone; those of the Rio Branco,
Jupura and Apoporis of quadersandstein; and those of the Amazon, on
several points, of ferruginous sandstone.* (* Braunes eisenschussiges
Sandstein-Conglomerat (Iron-sand of the English geologists, between
the Jura limestone and green sandstone.) MM. Spix and Martius found on
rocks of quadersandstein, between the Apoporis and the Japura, the
same sculptures which we have pointed out from the Essequibo to the
plains of Cassiquiare, and which seem to prove the migrations of a
people more advanced in civilization than the Indians who now inhabit
those countries.) It remains to examine if (as I am inclined to
suppose) the limestone and gypsum formations of the eastern part of
the littoral Cordillera of Venezuela differ entirely from those of the
Llanos, and to what series belongs that rocky wall* named the Galera,
which bounds the steppes of Calabozo towards the north? (* Is this
wall a succession of rocks of dolomite or a dyke of quadersandstein,
like the Devil's Wall (Teufelsmauer), at the foot of the Hartz?
Calcareous shelves (coral banks), either ledges of sandstone (effects
of the revulsion of the waves) or volcanic eruptions, are commonly
found on the borders of great plains, that is, on the shores of
ancient inland seas. The Llanos of Venezuela furnish examples of such
eruptions near Para(?) like Harudje (Mons Ater, Plin.) on the northern
boundary of the African desert (the Sahara). Hills of sandstone rising
like towers, walls and fortified castles and offering great analogy to
quadersandstein, bound the American desert towards the west, on the
south of Arkansas.) The basin of the steppes is itself the bottom of a
sea destitute of islands; it is only on the south of the Apure,
between that river and the Meta, near the western bank of the Sierra,
that a few hills appear, as Monte Parure, la Galera de Sinaruco and
the Cerritos de San Vicente. With the exception of the fragments of
tertiary strata above mentioned there is, from the equator to the
parallel of 10 degrees north (between the meridian of Sierra Nevada de
Merida and the coast of Guiana), if not an absence, at least a
scarcity of those petrifactions, which strikes an observer recently
arrived from Europe.
The maxima of the height of the different formations diminish
regularly in the country we are describing with their relative ages.
These maxima, for gneiss-granite (Peak of Duida in the group of
Parime, Silla de Caracas in the coast chain) are from 1300 to 1350
toises; for the limestone of Cumanacoa (summit or Cucurucho of
Turimiquiri), 1050 toises; for the limestone of Caripe (mountains
surrounding the table-land of the Guarda de San Augustin), 750 toises;
for the sandstone alternating with the limestone of Cumanacoa
(Cuchilla de Guanaguana), 550 toises; for the tertiary strata (Punta
Araya), 200 toises.
The tract of country of which I am here describing the geological
constitution is distinguished by the astonishing regularity observed
in the direction of the strata of which the rocks of different eras
are composed. I have already often pointed the attention of my readers
to a geognostic law, one of the few that can be verified by precise
measurements. Occupied since the year 1792 by the parallelism, or
rather the loxodromism of the strata, examining the direction and
inclination of the primitive and transition beds, from the coast of
Genoa across the chain of the Bochetta, the plains of Lombardy, the
Alps of Saint Gothard, the table-land of Swabia, the mountains of
Bareuth, and the plains of Northern Germany, I was struck with the
extreme frequency, if not the uniformity, of the horary directions 3
and 4 of the compass of Freiberg (direction from south-west to
north-east). This research, which I thought might lead to important
discoveries relating to the structure of the globe, had then such
attractions for me that it was one of the most powerful incentives of
my voyage to the equator. My own observations, together with those of
many able geologists, convince me that there exists in no hemisphere a
general and absolute uniformity of direction; but that in regions of
very considerable extent, sometimes over several thousand square
leagues, we observe that the direction and (though more rarely) the
inclination have been determined by a system of particular forces. We
discover at great distances a parallelism (loxodromism) of the strata,
a direction of which the type is manifest amidst partial perturbations
and which often remains the same in primitive and transition strata. A
fact which must have struck Palasson and Saussure is that in general
the direction of the strata, even in those which are far distant from
the principal ridges, is identical with the direction of mountain
chains; that is to say, with their longitudinal axis.
Venezuela is one of the countries in which the parallelism of the
strata of gneiss-granite, mica-slate and clay-slate, is most strongly
marked. The general direction of these strata is north 50 degrees
east, and the general inclination from 60 to 70 degrees north-west.
Thus I observed them on a length of more than a hundred leagues, in
the littoral chain of Venezuela; in the stratified granite of Las
Trincheras at Porto Cabello; in the gneiss of the islands of the lake
of Valencia, and in the vicinity of the Villa de Cura; in the
transition-slate and greenstone on the north of Parapara; in the road
from La Guayra to the town of Caracas, and through all the Sierra de
Avila in Cape Codera; and in the mica-slate and clay-slate of the
peninsula of Araya. The same direction from north-east to south-west,
and this inclination to north-west, are also manifest, although less
decidedly, in the limestones of Cumanacoa at Cuchivano and between
Guanaguana and Caripe. The exceptions to this general law are
extremely rare in the gneiss-granite of the littoral Cordillera; it
may even be affirmed that the inverse direction (from south-east to
north-west) often bears with it the inclination towards south-west.
As that part of the group of the Sierra Parime over which I passed
contains much more granite* than gneiss (* Only the granite of the
Baragon is stratified, as well as crossed by veins of granite: the
direction of the beds is north 20 degrees west), and other rocks
distinctly stratified, the direction of the layers could be observed
in this group only on a small number of points; but I was often struck
in this region with the continuity of the phenomenon of loxodromism.
The amphibolic slates of Angostura run north 45 degrees east, like the
gneiss of Guapasoso which forms the bed of the Atabapo, and like the
mica-slate of the peninsula of Araya, though there is a distance of
160 leagues between the limits of those rocks.
The direction of the strata, of which we have just noticed the
wonderful uniformity, is not entirely parallel with the longitudinal
axes of the two coast chains, and the chain of Parime. The strata
generally cut the former of those chains at an angle of 35 degrees,
and their inclination towards the north-west becomes one of the most
powerful causes of the aridity which prevails on the southern
declivity* of the mountains of the coast. (* This southern declivity
is however less rapid than the northern.) May we conclude that the
direction of the eastern Cordillera of New Grenada, which is nearly
north 45 degrees east from Santa Fe de Bogota, to beyond the Sierra
Nevada de Merida, and of which the littoral chain is but a
continuation, has had an influence on the direction (hor. 3 to 4) of
the strata in Venezuela? That region presents a very remarkable
loxodromism with the strata of mica-slate, grauwacke, and the
orthoceratite limestone of the Alleghenies, and that vast extent of
country (latitude 56 to 68 degrees) lately visited by Captain
Franklin. The direction north-east to south-west prevails in every
part of North America, as in Europe in the Fitchtelgebirge of
Franconia, in Taunus, Westerwald, and Eifel; in the Ardennes, the
Vosges, in Cotentin, in Scotland and in the Tarentaise at the
south-west extremity of the Alps. If the strata of rocks in Venezuela
do not exactly follow the direction of the nearest Cordillera, that of
the shore, the parallelism between the axis of one chain, and the
strata of the formations that compose it, are manifest in the Brazil
group.* (* The strata of the primitive and intermediary rocks of
Brazil run very regularly, like the Cordillera of Villarica (Serra do
Espinhaco) hor. 1.4 or hor. 2 of the compass of Freiberg (north 28
degrees east.))