Now Those Names Cannot Remain The Same If, After Further Examination, The Objects Of Comparison Have Not Retained The Same Place In The Geologic Series; If The Most Able Geologists Now Take For Transition-Limestone And Green Sandstone, What They Took Formerly For Zechstein And Variegated Sandstone.

I believe the surest means by which geologic descriptions may be made to survive the change which the science

Undergoes in proportion to its progress, will be to substitute provisionally in the description of formations, for the systematic names of red sandstone, variegated sandstone, zechstein and Jura limestone, names derived from American localities, as sandstone of the Llanos, limestone of Cumanacoa and Caripe, and to separate the enumeration of facts relative to the superposition of soils, from the discussion on the analogy of those soils with those of the Old World.*

(* Positive geography being nothing but a question of the series or succession (either simple or periodical) of certain terms represented by the formations, it may be necessary, in order to understand the discussions contained in the third section of this memoir, to enumerate succinctly the table of formations considered in the most general point of view.

1. Strata commonly called Primitive; granite, gneiss and mica-slate (or gneiss oscillating between granite and mica-slate); very little primitive clay-slate; weisstein with serpentine; granite with disseminated amphibole; amphibolic slate; veins and small layers of greenstone.

2. Transition strata, composed of fragmentary rocks (grauwacke), calcareous slate and greenstone, earliest remains of organized existence: bamboos, madrepores, producta, trilobites, orthoceratites, evamphalites). Complex and parallel formations; (a) Alternate beds of grey and stratified limestone, anthracitic mica-slate, anhydrous gypsum and grauwacke; (b) clay-slate, black limestone, grauwacke with greenstone, syenite, transition-granite and porphyries with a base of compact felspar; (c) Euphotides, sometimes pure and covered with jasper, sometimes mixed with amphibole, hyperstein and grey limestone; (d) Pyroxenic porphyries with amygdaloides and zirconian syenites.

3. Secondary strata, presenting a much smaller number of monocotyledonous plants; (a) Co-ordinate and almost contemporary formations with red sandstone (rothe todtes liegende), quartz-porphyry and fern-coal. These strata are less connected by alternation than by opposition. The porphyries issue (like the trachytes of the Andes) in domes from the bosom of intermediary rocks. Porphyritic breccias which envelope the quartzose porphyries. (b) Zechstein or Alpine limestone with marly, bituminous slate, fetid limestone and variegated gypsum (Productus aculeatus). (c) Variegated sandstone (bunter sandstein) with frequent beds of limestone; false oolites; the upper beds are of variegated marl, often muriatiferous (red marl, salzthon) with hydrated gypsum and fetid limestone. The gem-salt oscillates from zechstein to muschelkalk. (d) Limestone of Gottingen or muschelkalk alternating towards the top with white sandstone or brittle sandstein. (Ammonitis nodosus, encrinites, Mytilus socialis): clayey marl is found at the two extremities of muschelkalk. (e) White sandstone, brittle sandstein, alternating with lias, or limestone with graphites; a quantity of dicotyledonous mixed with monocotyledonous plants. (f) Jura limestone of complex formation; a quantity of sandy intercalated marl. We most frequently observe, counting from below upwards; lias (marly limestone with gryphites), oolites, limestone with polypi, slaty limestone with fish, crustacea, and globules of oxide of iron (Amonites planulatus, Gryphaea arcuata). (g) Secondary sandstone with lignites; iron sand; Wealden clay; greensand or green sandstone; (h) Chlorite; tufted and white chalk; (planerkalk, limestone of Verona.)

4. Tertiary strata, showing a much smaller number of dicotyledonous plants. (a) Clay and tertiary sandstone with lignites; plastic clay; mollasse and nagelfluhe, sometimes alternating where chalk is wanting, with the last beds of Jura limestone; amber. (b) Limestone of Paris or coarse limestone, limestone with circles, limestone of Bolca, limestone of London, sandy limestone of Bognor; lignites. (c) Silicious limestone and gypsum with fossil bones alternating with marl. (d) Sandstone of Fontainebleau. (e) Lacustrine soil with porous millstone grit. (e) Alluvial deposits.)

1. CO-ORDINATE FORMATIONS OF GRANITE, GNEISS AND MICA-SLATE.

There are countries (in France, the vicinity of Lyons; in Germany, Freiberg, Naundorf) where the formations of granite and gneiss are extremely distinct; there are others, on the contrary, where the geologic limits between those formations are slightly marked, and where granite, gneiss and mica-slate appear to alternate by layers or pass often from one to the other. These alternations and transitions appeared to me less common in the littoral Cordillera of Venezuela than in the Sierra Parime. We recognise successively, in the former of these two systems of mountains, above all in the chain nearest the coast, as predominating rocks from west to east, granite (longitude 70 to 71 degrees), gneiss (longitude 68 1/2 to 70 degrees), and mica-slate (longitude 65 3/4 to 66 1/2 degrees); but considering altogether the geologic constitution of the coast and the Sierra Parime, we prefer to treat of granite, gneiss and mica-slate, if not as one formation, at least as three co-ordinate formations closely linked together. The primitive clay-slate (urthonschiefer) is subordinate to mica-slate, of which it is only a modification. It no more forms an independent stratum in the New Continent, than in the Pyrenees and the Alps.

(a) GRANITE which does not pass to gneiss is most common in the western part of the coast-chain between Turmero, Valencia and Porto Cabello, as well as in the circle of the Sierra Parime, near the Encaramada, and at the Peak of Duida. At the Rincon del Diablo, between Mariara and Hacienda de Cura, and at Chuao, it is coarse-grained, and contains fine crystals of felspar, 1 1/2 inches long. It is divided in prisms by perpendicular vents, or stratified regularly like secondary limestone, at Las Trincheras, the strait of Baraguan in the valley of the Orinoco, and near Guapasoso, on the banks of the Atabapo. The stratified granite of Las Trincheras, giving birth to very hot springs (from 90.5 degrees centigrade), appears from the inclination of its layers to be superposed on gneiss which is seen further southward in the islands of the lake of Valencia; but conjectures of superposition founded only on the hypothesis of an indefinite prolongation of the strata are doubtful; and possibly the granite masses which form a small particular zone in the northern range of the littoral Cordillera, between 70 degrees 3 minutes and 70 degrees 50 minutes longitude, were upheaved in piercing the gneiss. The latter rock is prevalent, both in descending from the Rincon del Diablo southward to the hot-springs of Mariara, and towards the banks of the lake of Valencia, and in advancing on the east towards the group of Buenavista, the Silla of Caracas and Cape Codera.

Enter page number Go  Previous Next
Page 152 of 170
Words from 155293 to 156394 of 174507