Imágenes de página
PDF

Craters.

Our section represents three cases of Volcanic craters; the most simple (i. 5) rising through Granite, or stratified rocks, at the bottom of the sea, and accumulating craters, which, like those of Lipari and Stromboli, Sabrina, and Graham Islands, are occasionally formed in various parts of the ocean.* The second case is that of volcanoes, which, like Etna and Vesuvius, are still in action on the dry land, (i. 1. to i. 4.) The third is that of extinct volcanoes, like those in Auvergne, (h'. h".) which, although there exist no historical records as to the period of their last eruptions, show by the perfect condition of their craters, that they have been formed since the latest of those aqueous inundations, that have affected the Basalts and Tertiary strata, through which they have burst forth.

One great difference between the more ancient Basaltic eruptions and those of the Lava and Trachyte of existing volcanoes, is that the emission of the former, probably taking place under the pressure of deep water, was not accompanied by the formation of any permanent craters.

In both cases, the fissures through some of which these Eruptions may have issued, are abundantly apparent under

the upward passage of the Lava through fractures in the sordid Granite.

At Graveneire, near Clermont, a stream of Lava still retains the exact form, in which it issued through a fissure in the side of a mountain of Granite, and overflowed the subjacent valley. Most accurate representations of this, and many similar productions of Volcanic Eruptions from the Granite of this District may be seen in Mr. Poulett Scrope's inimitable Panoramic Views of the Volcanic formations of Central France.

* Within the last few years, the Volcanic Cones of Sabrina in the Atlantic, and of Graham Island in the Mediterranean, have risen suddenly in the sea and been soon levelled and dispersed by the Waves.

EXPLANATION OF PLATE i. 9

the form of Dikes, filled with materials similar to those which form the masses that have overflowed in the Vicinity of each Dike."

Changes offected by the Igneous Rocks, on the Strata in contact with them.

The peculiar condition of the rocks that form the side walls of Granitic Veins and Basaltic Dikes, affords another argument in favour of their igneous origin; thus wherever the early Slate rocks are intersected by Granitic Veins (a. 8.) they are usually altered to a state approximating to that of fine-grathed Mica-slate, or Hornblendeslate.

The secondary and Tertiary rocks also, when they are intersected by basaltic Dikes, have frequently undergone some change; beds of Shale and Sandstone are indurated, and reduced to Jasper; compact Limestone and Chalk are converted to crystalline Marble, and Chalk-flints altered to a state like that resulting from heat in an artificial furnace.f

In all these cases, the Phenomena appear to be throughout consistent with the theory of igneous Injection, and to be incapable of explanation on any other Hypothesis that has been proposed. A summary statement of the probable relations of the Granitic and Trappean Rocks to the other materials of the Globe, and to one another, may be found in De la Beche's Geological Researches, 1st Edit. Pag. 374,

[ocr errors]

* In many Dikes the materials have been varionsly modified, by their mode of cooling, and differ from the masses which overflowed the surface.

f Examples of this kind occur on the sides of Basaltic Dikes intersecting Chalk in the County of Antrim, and in the Island of Raghlin, See Geol. Trans. London, O. S. vol. iii. p. 210, pl. 10.

Explanation of Letters and Figures used in the references to unstratified and crystalline Rocks in Plate 1.

a. Granite. b. Sienite. c. Porphyry. d. Greenstone. e. Serpentine. f. Basalt, or Trap. g. Trachyte. h. Products of Extinct Volcanoes.

i. Products of Active Volcanoes.

a. 1.-a. 3. Mountains of Granite, raised into lofty ridges, from beneath Gneiss and Primary Slates. a. 4. Granite intermixed with Gneiss. a. 5–a. 8. Granite, subjacent to stratified rocks of all ages, and intersected by volcanic rocks. a. 9. Granite Veins, intersecting Granite, Gneiss, and - primary Slate. a. 10. Granite Vein, intersecting Primary and Transition rocks, and forming overlying masses at the surface. a. 11. Granite Wein intersecting Secondary strata, and overlying Chalk.” b. Dikes of Sienite. b. 1. Overlying masses of Sienite. c. Dikes of Porphyry. c. 1. Overlying masses of Porphyry. d. Dikes of ancient Greenstone. d. 1. Overlying masses of the same. The Rocks represented by d. and e. often pass into one another. e. Dikes of Serpentine. e. 1. Overlying masses of Serpentine. f. Dikes and intruded subterraneous masses of Basalt. f. 1. to f. 7. Masses of Basalt protruded through, and overlying strata of various ages.

* In the locality quoted in the Explanation of Plates, Vol. II. p. 5, the Granite which comes to the surface over the Chalk, is not covered

by Tertiary deposites, as represented in our section, Pl. 1.

[ocr errors]

Explan ATION OF PLATE I. 11

f. 8. Basaltiform products of Modern Volcanoes. g. Trachyte forming Dikes. g. 1. Trachyte forming overlying Domes. (Puy de Dome.) h. 1. h. 2. Lava of extinct Volcanoes, forming undisturbed Cones. (Auvergne.) i.—i. 5. Lava, Scoriae, and Craters of active Volcanoes. (i. 1–i. 4. Etna. 1-5. Stromboli.) k.—k. 24. Metalliferous Veins. k. 15'. Lateral expansions of Weins into metalliferous cavities, called by the Miners Pipe Veins, or Flats. l—l. 7. Faults, or fractures and dislocations of the strata. The continuity of stratified Rocks is always interrupted, and their level more or less changed on the opposite sides of a fault. It is unnecessary here to give detailed descriptions of the 28 divisions of the Stratified Rocks represented in our Section. Their usual Order of Succession and Names are expressed in their respective places, and detailed descriptions of their several characters may be found in all good Treatises on Geology. The leading Groups of Formations are united by colours, marking their separation from the adjacent groups; and the same colours are repeated, in the headings above the figures of Plants and Animals that characterize the several series of Formations, to show the extent of the strata over which the Organic Remains of each Group are respectively distributed. The Formation of Peat Bogs and Calcareous Tufa are of too local a nature to be included in the series of stratified Rocks represented in this Section; although they sometimes operate locally to a considerable extent, in adding permament and solid matter to the surface of the Globe.

List of the Names of the Plants and Animals, represented in Pl. 1. to denote the prevailing Types of Vegetable aud Animal Life, during the formation of the three great divisions of stratified Rocks.

REFERENCES.

r. recent, f fossil. Ad. B. Adolphe Brongniart. L. Lindley.

[ocr errors]

14. 15. 16. 17.

Ag Agassiz. P. Page of Vol. I.

Remains in Transition Strata.

- LAND PLANTS.

Araucaria. Norfolk Island Pine. r. & f. P. 364.
Equisetum. r. & f. P. 346.

. Calamites nodosus. f. (L. Pl. 16.)
. Asterophillites comosa. f. (L. 108.)
. Asterophyllites foliosa. f. (L. 25.)
. Aspidium. r. Pecopteris. f. -
. Cyathea glauca, Tree Fern. r. (Ad. B. Hist. Veg.

[ocr errors]

. Osmunda. r." Neuropteris. f.

. Lycopodium cernuum. r. (from Mirbel.) P. 350. 10. 11. 12. 13.

Lycopodium alopecuroides. r. (from Mirbel.) P. 350.
Lepidodendron Sternbergii. f. -
Lepidodendron gracile ! f.
Flabelliform Palm. r. (from Mirbel.) Palmacites. f.

MARINE ANIMALS AND PLANTS,

Acanthodes. f. Ag.

Catopterus. f. Ag.

Amblypterus. f. Ag.
Orodus, extinct Shark. f. (imaginary restoration.)

* An error in this figure represents the fructification as branching from the tallest frond, instead of rising by a separate rachis from the

root.

« AnteriorContinuar »