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Show 26 CAUSES OF' THE SUPERPOSITION [Ch.III. the circumstances under which the secondary and tertiary series originated, it is quite natural that particular tertiary groups should occupy areas of c.ompara~ively small extent,.that they should frequently coils1st of httoral and lacustrine deposits, and that they should ofte~ contain. those ~dmixtures of terrestrial, freshwater, and marme remams, which are so rare in secondary rocks. It might also be expected, that the tertiary volcanic formations should be much less exclusively submarine, and this we accordingly find to be the case. CAUSES OF THE SUPERPOSITION OF SUCCESSIVE FORMATIONS HAVING DISTINCT MINERAL AND ORGANIC CHARACTERS. But we have still to account for those remarkable breaks in the series of superimposed formations, which are common both to the secondary and tertiary rocks, but are more particularly frequent in the latter. The elucidation of this curious point is the more important, because geologists of a certain school appeal to phenomeiia of this kind in support of their doctrine of great catastrophes, out of the ordinary course of nature, and sudden revolutions of the globe. It is only by carefully considering the combined action of all the causes of change now in operation, whether in the animate or inanimate world, that we can hope to explain such compli· cated appearances as are exhibited in the general arrangement of mineral masses. In attempting, therefore, to trace the ol'igin of these violations of continuity, we must re-consider many of the topics treated of in our two former volumes, such as the effects of the various agents of decay and reproduction, the imbedding of organic remains, and the extinction of species. Shifting of the Areas of Sedimentary Deposition.-By re· verting to our survey of the destroying and renovating agents, it will be seen that the surface of the terraqueous globe may be divided into two parts, one of which is undergoing repair, while the other, constituting, at any one period, by far the Ch. III.] OF SUCCESSIVE FORMATIONS. 27 largest portion of the whole, is either suffering degradation, or remaining stationary without loss or increment. Tlte reader will assent at once to this proposition, when he reflects that the dry land is, for the most part, wasting by the action of rain, rivers, and torrents, while the effects of vegetation have, as we have shown, only a conservative tendency, being very rarely instrumental in adding new masses of mineral matter to the surface of emerged lands; and when he also reflects that part of the bed of the sea is exposed to the excavating action of currents, while the gt·eater part, remote from continents and islands, probably receives no new deposits whatever, being covered for ages with the clear blue waters uncharged with sediment. Here the relics of organic beings, lying in the ooze of the deep, may decompose like the leaves of the forest in autumn, and leave no wreck behind, but merely supply nourishment, by their decomposition, to succee~ing races of marine animals and plants. The other part of the terraqueous surface is the receptacle of new deposits, and in this portion alone, as we pointed out in the last volume, the remains of animals and plants become fossilized. Now the position of this area, where new formations are in progress, and where alone any memorials of the state of organic life are preserved, is always varying, and must for ever continue to vary; and, for the same reason, that portion of the tcn·aqueous globe which is undergoing waste, also shifts its position, and these fluctuations depend partly on the action of aqueous, and partly of igneous causes. In illustration of these positions we may observe, that the sediment of the Rhone, which is thrown into the lake of Ge· neva, is now conveyed to a spot a mile and a half distant from that where it accumulated in the tenth century, and six miles from the point where the delta began originally to form. We may look forward to the period when the lake will be filled up, and then a sudden change will take place in the distribution of the transported matter; for the mud and sand brought down from the Alps wlll thenc~forth~ instead of being deposited I . |