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Show 46 CLASSIFICATION OF TERTIARY FORMATlONS [Ch. V. lected an d compare d wi th those of other. countries, and w.e ar. e almost entt·r e1 Y 1· g, nora nt of many deposits known to exist m Spain and Portugal. h t. 1 ·1The t eore tea v ews developed in t. he last chapter, res. pect· m. g brea k s m· tl1 e se quence of geolocon c. al monume.n ts, Will .e x- plam. our reasons ~or· anticipating the dtscovery of mtermed1ate 1' gra d atw· ns as of t en as new recbr ions of great extent are explored. C omp ara tt.v e value oif di.fvf e· rent classes of organic remains. I n t h e mean tI' me , ,ve mw;t:' t endeavour to make t•h e most• syste- mau·c arrange ment I'n our power of those fo.r mations .w hiCh at·e a1 r ea d y k nown, and in attemptinOo' to classify these m chrono- 1o gi·C a 1 or d er, '"" e have already stated t.h at w.e must . chiefly depend on the evidence affor~ed by their fossil orgamc ~on-tents. In the execution of this task, we have first to consider what class of remains are most useful, for although every kind of fossil animal and plant is interesting, and cannot fail to throw light on the former history of th~ globe at a certain period, yet those classes of remains which are of rare and casual occurrence, are absolutely of no use for the purposes of general classification. If we have nothing but plants in one assemblage of strata and the bones of mammalia in another, we can ob- viously d'r aw no conclusion respecting the number of speci.e s of organic beings common to two epochs ; or if we have a great variety, both of vertebrated animals and plants, in one series, and only shells in another, we can form no opinion respecting the remoteness or proximity of the two eras. We might, perhaps, draw some conclusions as to relative antiquity, if we could compare each of these monuments to a third; as, for example, if the ~pecies of shells should be almost all identical with those now living, while the plants and vertebrated animals were all extinct; for we might then infer that the shelly deposit was the most recent of the two. But in this case it will be seen that the information flows from a direct comparison of the species of corresponding orders of the animal and vegetable kingdoms,of plants with plants, and shells with shells; the only mode of Ch. V.] tN CHRONOLOGICAL ORDER. making a systematic arrangement by reference to organic remains. Although the bones of mammalia in the tertiary strata, and those of reptiles in the secondary, afford us instruction of the most interesting kind; yet the species are too few, and confined to too small a number of localities, to be of great importance in characterizing the minor subdivisions of geological formations4 Skeleton of fish are by no means frequent in a good state of preservation, and the science of ichthyology must be farther nd vanced, before we can hope to determine their specific character with sufficient precision. The same may be said of fossil botany, notwithstanding the great progress that has recently been made in that department ; and even· in regard to zoophytes~ which are so much more abundant in a fossil state than any of the classes above enumerated, we are still greatly impeded in our endeavour to classify strata by their aid, in consequence of the smallness of the number of recent species which have been examined in those tropical seas where they occur in the greatest profusion. Fossil remains of testacea of chief importance. The testacea are by far the most important of all classes of organic beings which have left their spoils in the subaqueous deposits; they are the medals which nature has chiefly selected to record the history of the for-mer changes of the globe. There is scarcely any great series of strata that does not contain some marine or freshwater shells, and these fossils are often found so entire, especially in the tertiary formations, that when disengaged from the matrix, they have all the appearance of having been just procured from the sea. Their colour, indeed, is usually wanting, but the parts whereon specific characters are founded remain unimpaired j and although the animals themselves are gone, yet their form and habits can generally be inferred from the shell which covered them. The utility of the testacea, in geological classification, is greatly enhanced by the circumstance, that some forms are proper to the sea, others 'to the land, and others to freshwater. |