Изображения страниц

Thus it appears certain that the variety of life, estimated by the marine tribes existing in a given period, is greater in the more recent periods; but the number of individuals, or the abundance of life, is not measured by the same proportions. Periods of extraordinary abundance alternate in every great series of strata with other periods of comparative scarcity; and though sometimes this may be explained by the well-known fact that red peroxide of iron in sedimentary strata is very unfavourable to marine invertebral life, while grey protoxidated rocks of the same series contain organic remains in abundance; and sometimes requires attention to the unequal conservative conditions, or originally unequal feeding circumstances of Calcareous, Argillaceous and Arenaceous sea-bottom; still it is a very impressive phenomenon in the continuously grey Cambrians and Silurians, in the continuously grey Carboniferous rocks, in the continuously protoxidated Oolitic strata, and in the almost uniform deposits of Lias, Oxford Clay, Kimmeridge Clay, Gault, London Clay and Barton Clay. An illustration is subjoined from the Lower Palæozoic Strata of Britain. Fig. 3.

[blocks in formation]

In this diagram the Passage Beds are taken at 500 ft. with 13 species, Ludlow 2000 ft. with 228 species, Wenlock 2500 ft. with 322 species, Llandovery 2500 ft. with 196 species, Caradoc 6800 ft. with 335 species, Llandeilo 6800 ft. with 104 species, Lingula beds 4000 ft. with 14 species. The area of the spaces corresponds to the number of species, thus shewing at a glance the relative richness in species of the several groups for equal thicknesses. This richness (expressed by numbers on the right side) rises from almost zero in the Lingula beds to a maximum in the upper part of the Wenlock series, and dies out almost to zero in the beds of Passage to the Old Red Sandstone system above.

What is here said of the Siluro-Cambrian or Lower Palæozoic Strata may be repeated with equal truth in reference to each of the systems of associated deposits: for in each the characteristic and prevalent fauna begins at a minimum, rises to a maximum, and dies away to a final minimum, to be followed by another system having similar phases. The most remarkable and prevalent of these surfaces or zones of least life are those two which separate the Palæozoic from the Mesozoic, and these from the Cænozoic Series. The Palæozoic Series dies out through the Permian system, and the Mesozoic rises slowly in the Trias; so the Mesozoic Series dies away in the uppermost


beds of the Cretaceous system, and the Cænozoic Series grows slowly through the lowest Eocene to a maximum in the later tertiaries.

A depression in the maximum occurs in the Palæozoic Series of Britain corresponding to the Devonian period, here so largely represented by peroxidated sediments; another in the Mesozoic period, in the uppermost Oolites, which are poor in comparison with the richer series of Oxford and Bath. These peculiarities are represented in the following diagram by a continuous curve, which corresponds to the numerical prevalence of life, and represents its rise and fall. Fig. 4.


R. L.

[blocks in formation]
« ПредыдущаяПродолжить »