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ExPLANATION OF PLATE i. 3
As it would encumber the section to express Diluvium, wherever it is present, it is introduced in one place only, which shows its age to be more recent than the newest of the Tertiary strata; it is found also lodged indiscriminately upon the surface of rocks of every formation.
In our early Chapters we have considered the Theory which refers unstratified rocks to an igneous Origin, to be that which is most consistent with all the known Phenomena of Geology, and the facts represented in the Section now before us are more consistent with the Postulates of this Hypothesis, than with those of any other that has hitherto been proposed. I have, therefore, felt it indispensable to adopt its language, as affording the only terms by which the facts under consideration can be adequately described. Assuming that Fire and Water have been the two great Agents employed in reducing the surface of the globe to its actual condition, we see, in repeated operations of these agents, causes adequate to the production of those irregular Elevations and Depressions of the fundamental Rocks of the Granitic series, which are delineated in the lower Region of our Section, as forming the basis of the entire Superstructure of stratified Rocks. Near the right extremity of this Section, the undulating surface of the fundamental Granite (a. 5. a. 6. a. 7. a. 8) is represented as being, for the most part, beneath the level of the Sea. On the left extremity of the Section (a. 1. a. 2. a. 3.) the Granite is elevated into one of those lofty Alpine ridges, which have affected, by their upward movement, the entire series of stratified Rocks. Corresponding formations of Primary and Transition Strata, are represented as occurring on each side of this elevated Granite, which is supposed to have broken through, and to have carried up with it to their present clevated and highly inclined position, strata that were once continuous and nearly horizontal.” The general history of Elevation appears to be, that mountain chains of various extent, and various directions, have been formed at irregular intervals, during the deposition of stratified rocks of every age; and that Granite had, in many cases, acquired a state of solidity before the period of its elevation. Within the primary Granite, we find other forms of Gramitic matter, (a. 9.) which appear to have been intruded in a state of fusion, not only into fissures of the older Granite, but frequently also into the Primary stratified rocks in contact with it, and occasionally into strata of the Transition and Secondary series, (a. 10. a. 11.) these Granitic injections were probably in many cases, contemporaneous with the elevation of the rocks they intersect; they usually assume the Condition of Veins, terminating upwards in small branches; and vary in dimensions, from less than an inch, to an indefinite width. The direction of these veins is very irregular: they sometimes traverse the Primary strata at right angles to their planes of stratification, at other times they are protruded in a direction parallel to these planes, and assume the form of beds. Some of the relations of these Granitic Weins to the rocks intersected by them are represented at the left extremity of the Section. (a. 9.) f
* Cases of Granite, thus elevated at a period posterior to the deposition of Tertiary Strata, occur in the Eastern Alps, where the Transition, Secondary, and Tertiary strata have all partaken of the same elevation which raised the central axis of the crystalline Granitic rocks. See Geol. Trans. N. S. Vol. III. Pl. 36. Fig. 1.
# In the Granite at the right extremity of the Section, the granitic veins are omitted, because their insertion would interfere with
ExPLANATION OF PLATE i. 5
A. 10. represents a dike and protruded mass of Granite, intersecting and overlying stratified rocks of the Primary and Transition series. A. 11. represents the rare case of Granite intersecting Red Sandstone, Oolite, and Chalk.”
Sienite, Porphyry, Serpentine, Greenstone.
Closely allied to Granite Veins, is a second series of irregularly injected rocks, composed of Sienite, Porphyry, Serpentine, and Greenstone (b. c. d. e.) which traverse the Primary and Transition formations, and the lower regions of the Secondary strata; not only intersecting them in various directions, but often forming also overlying masses, in places where these veins have terminated by overflowing at the surface, (b'. c'. d’. e.) The crystalline rocks of this series, present so many modifications of their ingredients, that numerous varieties of Sienite, Porphyry, and Greenstone occur frequently in the products of Eruptions from a single vent.
The scale of our Section admits not of an accurate representation of the relations between many of these intruded rocks, and the strata they intersect; they are all placed, as
the representation of the injections of Basaltic and Volcanic matter which the portion of the section is intended to illustrate. * An example of the rare Phenomenon of Granite intruded into the Chalk formation, in the hill of St. Martin, near Pont de la Fou in the Pyrenees, is described by M. Dufrenoy in the Bulletin de la Société Géologique de France, Tom. 2. p. 73. At Weinböhla, near Meissen in Saxony, Prof. Weiss has ascertained the presence of Sienite above strata of Chalk; and Prof. Nauman states, that, near Oberau, Cretaceous rocks are covered by Granite, and that near Zscheila and Neiderfehre, the Cretaceous rocks rest horizontally on Granite; at both these places the Limestone and Granite are entangled in each other, and irregular portions and veins of hard Limestone, with green grains and cretaceous fossils, are here and there imbedded in the Granite. De la Beche. Geol. Manual. 3d Edit. p. 295.
if they had been injected, either at the time of, or after the elevation of all the strata, and had produced but little disturbance in the rocks through which they are protruded. It should however be understood, distinctly, that some Injections may have preceded the elevation of Strata to their present height, and that numerous and successive elevations and injections, attended by various degrees of fracture and disturbance, have prevailed in various localities during all periods, and throughout all formations; from the first upraising of the earliest Primary rocks, to the most recent movements produced by existing Volcanoes. M. Elie de Beaumont has discovered probable evidence of no less than twelve periods of elevation, affecting the strata of Europe. Examples of the fractures and dislocations attending these movements, and producing faults, are represented in our Section by the lines designated by the letter 1. Some of these fractures do not reach to the present surface, as they affected the lower beds at periods anterior to the deposition of more recent strata, which cover unconformably the summits of the earlier fractures. (See l. l'. I*. 1", 1". 17.)
A third series of Igneous Rocks is that which has formed dikes, and masses of Basalt and Trap, intruded into, and overlying formations of all ages, from the earliest Granites to the most recent Tertiary Strata. These basaltic rocks sometimes occur as Beds, nearly parallel to the strata, into which they are protruded, after the manner represented in the carboniferous Limestone of our Section, f. 2. More frequently they overspread the surface like expanded sheets of Lava. Our section gives examples of Trap under all these circumstances. At f. 1. it intersects and overlies Primary strata; at f. 2. f. 3. f. 4. s. 5. it stands in similar
EXPLANATION OF Plate i. 7
relations to Transition and Secondary strata; f. 6. represents an example of an extensive eruption of Basaltic matter, over Chaik and Tertiary strata, accompanied by an intrusion of vast irregular masses of the same materials into the body of the subjacent Primary and Transition rocks. f. 7. represents strata of columnar Basalt, immediately beneath streams of cellular Lava, in regions occupied also by craters of extinct Volcanoes. f. S. represents similar beds of columnar lava in the vicinity of active Volcanoes.
Trachyte and Lava.
The fourth and last class of intruded rocks, is that of modern volcanic Porphyries, Trachytes,” and Lavas. The undeniable igneous origin of rocks of this class forms the strongest ground-work of our arguments, in favour of the igneous formation of the older unstratified and crystalline rocks; and their varied recent products, around the craters of active Volcanoes, present gradations of structure, and composition, which connect them with the most ancient Porphyries, Sienites, and Granites.
The simplest cases of volcanic action are those of Trachyte (g. 1) and of Lava (i. 5) ejected through apertures in Granite; such cases prove that the source of volcanic fires, is wholly unconnected with the pseudo-volcanic results of the combustion of coal, bitumen, or sulphur, in stratified formations, and is seated deep beneath the Primary rocks.f
* The appellation of Trachyte has been given to a volcanic Porphyry, usually containing Crystals of glassy felspar, and remarkably harsh to the touch, (hence its name from teaxvi); it does not occur in Britain, but abounds in the neighbourhood of almost all extinct and active volcanic craters.
# The occurrence of angular fragments of altered Granite, embedded in Pillars of columnar Lava, in the valley of Monpezat in the Ardèche, shows that these fragments were probably torn off during