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Show 90 UNIFORMITY IN TilE course of so many ages · In addition to volca.n ic heat, to which the V ulcanists formerly attributed too much mfluen~e, we ~ust 11 ./! • th effect of mechanical pressure, of chemtcal affimty, a OW .LOr e • b l ' of percolation by mineral waters, of permeat10n y e ast1c fl 'd d the action perhaps, of many other forces less under- Ul s, an ' . d h · stoo d , sue h as electricity and magnetism. In regar · tot· e s1gkn s of upraising and sinking, of fracture and contort10n m roc s, it is evident that newer strata cannot be shaken by earth-k S qua e, unless the sub1acent rocks are also affected ; so that the J • b . 1 contrast in the relative degree of d1stur ance m t 1e more ancient and the newer strata, is one of many proofs that the convulsions have happened in different eras, and the fa~t confirms the uniformity of the action of subterranean forces, mstead of their greater violence in the ~rimeval ages .. 'rhe popular doctrine of umversal ~o~mat~ons, or th~ ~nlimited geographical extent of strata, d1s~mgmshed by s~m1lar mineral characters, appeared for a long tlme to present msurmountable objections to the supposition, that the earth's crust had been formed by causes now acting. If _it had merely been assumed, that rocks originating from fusiOn by subterranean fire presented in all parts o~ .the ~globe a pe~fect correspondence in their mineral compositlOn, the assumption would not have been extravagant; for, as the elementary substances that enter largely into the composition of rocks are few in number, they may be expected to arrange themselves i~variably in the same forms, whenever the elementary particles are freely exposed to the action of che~ical a~nities: Bu~ when it was imagined that sedimentary mixtures, mcludmg ammal ~nd vegetable remains, and evidently formed in the beds of anc1e1~t seas, were of homogeneous nature throughout a whole hemlsphere, or even farther, the dogma precluded at once. all hope of recognizing the slightest analogy between the ancient and modern causes of decay and reproduction. For we k~ow t~at existing rivers carry down from different mountam-chams sediment of distinct colours and composition; where the chains are near the sea, coarse sand and gravel is swept in; where they are distant, the finest mud. We know'. also, tha.t the matter introduced by springs into lakes and seas Is very diversified in mineral composition ; in short, contempora~eou~ stra~ now in the progress of formation are greatly var1ed m thell' ORDER OF NATURE. 91 composition, and could never afford formations of homogeneous minera] ingredients co-extensive with the greater part of the earth's surface. This theory, however, is as inapplicable to the effects of those operations to which the formation of the earth's crust is due, as to the effects of existing causes. The first investigators of sedimentary rocks had never reflected on the great areas occupied by modern deltas of large rivers; still less on the much greater areas over which marine currents, preying alike on river-deltas, and continuous lines of sea-coast, might be diffusing homogeneous mixtures. They were ignorant of the vast spaces over which calcareous and other mineral springs abound upon the land and in the sea, especially in and near volcanic regions, and of the quantity of matter discharged by them. When, therefore, they ascertained the extent of the geographical distribution of certain groups of ancient stratawhen they traced them continuously from one extremity of Europe to the other, and found them flanking, throughout their entire range, great mountain-chains, they were astonished at so unexpected a discovery ; and, considering themselves at liberty to disregard all modern analogy, they indulged in the sweeping generalization, that the law of continuity prevailed throughout strata of contemporaneous origin over the whole planet. The difficulty of dissipating this delusion was extreme, because some rocks, formed under similar circumstances at different epochs, present the same external characters, and often the same internal composition; and all these were assumed to be contemporaneous until the contrary could be shown, which, in the absence of evidence derived from direct superposition, and in the scarcity of organic remains, was often impossible. Innumerable other false generalizations have been derived from the same source; such, for instance, as the former univerS!). lity of the ocean, now disproved by the discovery of the remains of terrestrial vegetation, contemporary with every successive race of marine animals; but we shall dwell no longer on exploded errors, but proceed at once to contend against weightier objections, which will require more attentive consideration. |