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Show 330 SECONDARY FORMATIONS. [Ch. XXIII. for more than 170 miles, and occupies, it is supposed, a space of more than 300 miles along the coast, thus forming a surface of more than 25,000 square miles, or equal to about one half of England *. Now if this modern 'delta,' or, in other words, that part of the bed of the Atlantic which has been converted into land by matter deposited immediately at the river's mouth, be so extensive, how much larger may be the space over which the same kind of sediment may be distributed by the action of the tides and currents ! If, then, groups like the Wealden may be formed near the mouths of great rivers, others, like the lias, may be pro~uced by the wider dispersion of similar materials over larger submarine areas. For we may conceive that the Niger may carry out the remains of land plants, and the carcasses and bones of fluviatile reptiles, into places where they may be swept away by currents and afterwards mingled far and wide with the marine shells and corals of the Atlantic. The reader will remember that we stated, in the first volumet, that the common crocodile of the Ganges frequents both fresh and salt water, the same species being sometimes seen far inland, many hundred miles from the sea, and at the ~arne time swarmin()' on the sand-banks in the salt and brack1sh water beyond 0 the limits of the delta. If we are asked where the continent was placed from the ruins of which the Wealden strata were derived, we are almost tempted to ~peculate on the former existctlce of the Atlantis of Plato, which may be true in geology, although fabulous as an historical event. We know that the present European lands have come into existence almost entirely since the depo· s1· t1· 0n o f the chalk ' and the same period may ha.v e suffic. ed for the disappearance of a continent of equal magmtude, situated farther to the west. Secondary fresh-water deposits why rare .. -If there were extensive tracts of land in the secondary perwd, we may pre· sume that there were lakes also; yet we are not aware of any * Fitton's Geology of Hastings, P· 58, who cites Lander's Travels. t Page 243; Second Edition, P• 279. Ch. XXIII.] PERSISTENCY OP' MINERAL CIIARACTER. 331 pure Jacustrine formations interstratified with rocks older than the chalk. Perhaps their absence may be accounted for by the adoption of the theoretical views above set forth ; for if the present ocean coincides for the most part with the site of the ancient continent, the places occupied by lakes must have been submerged. It should also be recollected, that the area covered by lakes, at any one time, is very insignificant in proportion to the sea, and, therefore, we may expect that, after the earth's surface has undergone considerable revolutions in its physical geography, the lacustrine strata will be concealed, for the most part, under superimposed marine deposits. Persistency of mineral character.-In the same manner as it is rare and difficult to find ancient lacustrine strata, so also we can scarcely expect to discover newer marine groups preserving the same lithological characters continuously throughout wide areas. The chalk now seen stretching for thousands of miles over different parts of Europe, has become visible to us by the effect, not of one, but of many distinct series of movements. Time has been required, and a succession of geological periods, to raise it above the waves in so many regions ; and if calcareous rocks of the Eocene or Miocene periods have been formed, preserving an homogeneous mineral composition throughout equally extensive regions, it may require convulsions as numerous as all those which have occurred since the origin of the chalk, to bring them up within the sphere of human observation.. Hence the rocks of more modern periods may appear of partial extent, as compared to those of remoter eras, not because there was any original difference of circumstances throughout the globe when they were formed, but because there has not been sufficient time for the development of a great series of subterranean volcanic operations since their origin. . A~ ~he ~arne time, the reader should be warned not to place ImpliCit rehance on the alleged persistency of the same mineral characters in secondary rocks*. When it was first ascertained that an order of succession could be traced in the principal • See some remarks on this subject, vol. i,ll• 90, and Second Edition, p. 102. |