CHAPTER IV.
Attempts to Discover General Laws in Geology.
Sect. 1.—General Geological Phenomena.
BESIDES thus noticing such features in the rocks of each country as were necessary to the identification of the strata, geologists have had many other phenomena of the earth’s surface and materials presented to their notice; and these they have, to a certain extent, attempted to generalize, so as to obtain on this subject what we have elsewhere termed the Laws of Phenomena, which are the best materials for physical theory. Without dwelling long upon these, we may briefly note some of the most obvious. Thus it has been observed that mountain ranges often consist of a ridge of subjacent rock, on which lie, on each side, strata sloping from the ridge. Such a ridge is an Anticlinal Line, a Mineralogical Axis. The sloping strata present their Escarpements, or steep edges, to this axis. Again, in mining countries, the Veins which contain the ore are usually a system of parallel and nearly vertical partitions in the rock; and these are, in very many cases, intersected by another system of veins parallel to each other and nearly perpendicular to the former. Rocky regions are often intersected by Faults, or fissures interrupting the strata, in which the rock on one side the fissure appears to have been at first continuous with that on the other, and shoved aside or up or down after the fracture. Again, besides these larger fractures, rocks have Joints,—separations, or tendencies to separate in some directions rather than in others; and a slaty Cleavage, in which the parallel subdivisions may be carried on, so as to produce laminæ of indefinite thinness. As an example of those laws of phenomena of which we have spoken, we may instance the general law asserted by Prof. [538] Sedgwick (not, however, as free from exception), that in one particular class of rocks the slaty Cleavage never coincides with the Direction of the strata.
The phenomena of metalliferous veins may be referred to, as another large class of facts which demand the notice of the geologist. It would be difficult to point out briefly any general laws which prevail in such cases; but in order to show the curious and complex nature of the facts, it may be sufficient to refer to the description of the metallic veins of Cornwall by Mr. Carne;[56] in which the author maintains that their various contents, and the manner in which they cut across, and stop, or shift, each other, leads naturally to the assumption of veins of no less than six or eight different ages in one kind of rock.
[56] Transactions of the Geol. Soc. of Cornwall, vol. ii.
Again, as important characters belonging to the physical history of the earth, and therefore to geology, we may notice all the general laws which refer to its temperature;—both the laws of climate, as determined by the isothermal lines, which Humboldt has drawn, by the aid of very numerous observations made in all parts of the world; and also those still more curious facts, of the increase of temperature which takes place as we descend in the solid mass. The latter circumstance, after being for a while rejected as a fable, or explained away as an accident, is now generally acknowledged to be the true state of things in many distant parts of the globe, and probably in all.
Again, to turn to cases of another kind: some writers have endeavored to state in a general manner laws according to which the members of the geological series succeed each other; and to reduce apparent anomalies to order of a wider kind. Among those who have written with such views, we may notice Alexander von Humboldt, always, and in all sciences, foremost in the race of generalization. In his attempt to extend the doctrine of geological equivalents from the rocks of Europe[57] to those of the Andes, he has marked by appropriate terms the general modes of geological succession. “I have insisted,” he says[58] “principally upon the phenomena of alternation, oscillation, and local suppression, and on those presented by the passages of formations from one to another, by the effect of an interior developement.”
[57] Gissement des Roches dans les deux Hemisphères, 1823.
[58] Pref. p. vi.