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.))

SECTION 3.

NATURE OF THE ROCKS. RELATIVE AGE AND SUPERPOSITION OF THE FORMATIONS. PRIMITIVE, TRANSITION, SECONDARY, TERTIARY, AND VOLCANIC STRATA.

The preceding section has developed the geographical limits of the formations, the extent of the direction of the zones of gneiss-granite, mica-slate-gneiss, clay-slate, sandstone and intermediary limestone, which come successively to light. We will now indicate succinctly the nature and relative age of these formations. To avoid confounding facts with geologic opinions I shall describe these formations, without dividing them, according to the method generally followed, into five groups - primitive, transition, secondary, tertiary and volcanic rocks. I was fortunate enough to discover the types of each group in a region where, before I visited it, no rock had been named. The great inconvenience of the old classification is that of obliging the geologist to establish fixed demarcations, while he is in doubt, if not respecting the spot or the immediate superposition, at least respecting the number of the formations which are not developed. How can we in many circumstances determine the analogy existing between a limestone with but few petrifactions and an intermediary limestone and zechstein, or between a sandstone superposed on a primitive rock and a variegated sandstone and quadersandstein, or finally, between muriatiferous clay and the red marl of England, or the gem-salt of the tertiary strata of Italy? When we reflect on the immense progress made within twenty-five years in the knowledge of the superposition of rocks, it will not appear surprising that my present opinion on the relative age of the formations of Equinoctial America is not identically the same with what I advanced in 1800. To boast of a stability of opinion in geology is to boast of an extreme indolence of mind; it is to remain stationary amidst those who go forward. What we observe in any one part of the earth on the composition of rocks, their subordinate strata and the order of their position are facts immutably true, and independent of the progress of positive geology in other countries; while the systematic names applied to any particular formation of America are founded only on the supposed analogies between the formations of America and those of Europe.

Enter page number Go  Previous Next
Page 151 of 170
Words from 154268 to 155292 of 174507