Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/338

Rh 324 most favourable conditions, only a small proportion of the total flora and fauna. of :my period could be preserved in the fossil state, enormous gaps occur where no record has been prcservel at all. It is as if whole chapters and books were missing from an historical work. Some of these lacunae are sufficiently obvious. Thus, in some cases, powerful disloca- tions have thrown considerable portions of the rocks out of sight. Sometimes extensive metamorphism has so affected them that their original characters, including their organic contents, have been destroyed. Oftenest of all, denudation has come into play, and vast masses of fossiliferous rock have been entirely worn away. That this cause has operated frequently is shown by the abundant unconformabilities in the structure of the earth’s crust. While the mere fact that one series of rocks lies uncon- fornnbly on another proves the lapse of a considerable interval between their respective dates, the relative length of this interval may sometimes be demonstrated by means of fossil evidence and by this alone. Let us suppose, for example, that a certain group of formations has been dis- turbed, npraised, denuded, and covered nnconformably by a second group. In lithological characters the two may closely resemble each other, and there may be nothing to show that the gap represented by their unconformability is not of a trifling character. In many cases, indeed, it would be quite impossible to pronounce any well-grounded judg- ment as to the amount. of interval, even measured by the vague elative standards of geological chronology. But if each group contains a well-preserved suite of organic re- mains, it may not only be possible, but easy, to say exactly how much of the geological record has been left out between the two sets of formations. By comparing the fossils with those obtained from regions where the geological record is more complete, it may be ascertained perhaps that the lower rocks belong to a certain platform or stage in geologi- cal history which for our present purpose we may call D, and that the upper rocks can in like manner he paralleled with stage II. It would be then apparent that at this locality the chronicles of three great geological periods E, F, and G were wanting, which are elsewhere found to be intercilated between D and H. The lapse of time repre- sented by this unconformability would thus be equivalent to that required for the accumulation of the three missing formations in those regions where sedimentation went on undisturbed. But fossil evidence may be made to prove the existence of gaps which are not otherwise apparent. As has been already remarked, changes in organic forms must, on the whole, have been extremely slow in the geological past. The whole species of a sea-floor could not pass entirely away, and be replaced by other’forms, without the lapse of long periods of time. If then among the conformable stratiﬁed formations of former ages we encounter sudden and abrupt changes in the facies of the fossils, we may be certain that these must mark omissions in the record, which we may hope to ﬁll in from a more perfect series elsewhere. The complete contrasts between unconformable strata are sufﬁciently explicable. It is not so easy to give a satisfactory account of those which occur where the beds are strictly conformable, and where no evidence can be observed of any considerable change of physical conditions at the time of deposit. A group of strata having the same general lithological characters throughout may be marked by a great discrepance between the fossils above and below a certain line. A few species may pass from the one into the other, or perhaps every species may be different. In cases of this kind, when proved to be not merely local but persistent over wide areas, we must admit, notwithstanding the apparently undisturbed and continuous character of the original deposition of the strata, that the abrupt transition G 1'} O L 0 G Y [v. P.-XL.-EONTOLOGICAL. from the one facies of fossils to the other must represent a long interval of time which has not been recorded by the ‘ deposit of strata. Professor ltamsay, who called attention to these gaps, termed them “breaks in the succession of organic remains.” He showed that they occur abundantly among the Palzeozoic and Secondary rocks of England. lt is obvious, of course, that these breaks, even though trace- able over wide regions, were not general over the whole globe. There have never been any universal interruptions in the continuity of the chain of being, so far as geological evidence can show. But the physical changes which caused the breaks may have been general over a zoological district or minor region. They no doubt often caused the com- plete extinction of genera and species which had a small geographical range. From all these facts it is clear that the geological record, as it now exists, is at the best but an imperfect chronicle of geological history. In no country is it complete. The lacunae of one region must be supplied from another. Yet in proportion to the geographical distance between the localities where the gaps occur and those whence the mis- sing intervals are supplied, the element of uncertainty in our reading of the record is increased. The most desirable method of research is to exhaust the evidence for each area or province, and to compare the general order of its succes- sion as a whole with that which can be established for other provinces. It is, therefore, only after long and patient observation and comparison that the geological history of different quarters of the globe can be correlated. S'ubtl£visions oft/ze Geological Ifecord by means 0_fI"0ssi/s. — As fossil evidence furnishes a 1nuch 111ore satisfactory and widely applicable means of subdividing the stratified rocks of the earth’s crust than mere lithological characters, it is made the basis of the geological classification of these rocks. Thus we may ﬁnd a particular stratum marked by the occurrence in it of various fossils, one or more of which may be distinctive, either from occurring in no other bed above and below, or from special abundance in that stratum. These species might therefore be used as a guide to the oc- currence of the bed in question, which might be called by the name of the most abundant species. In this way a Geological horizon or zone would be marked off, and geologists would thereafter recognize its exact position in the series of forma- tions. But before such a generalization can be safely made, we must be sure that the species in question really never does appear on any other platform. This evidently demands wide experience over an extended ﬁeld of observation. The assertion that a particular species occurs only on one horizon manifestly rests on negative evidence as much as on positive. The palaeontologist who makes it cannot mean more than that he knows the fossil to lie on that horizon, and that, so far as his own experience and that of others goes, it has never been met with anywhere else. But a single example of the occurrence of the fossil on a different zone would greatly damage the value of his generalization, aml a few such cases would demolish it altogether. Hence all such statements ought at first to be made tentatively. To estab- lish a geological horizon on limited fossil evidence, and then to assume the identity of all strata containing the same fossils, is to reason in a circle and to introduce utter con- fusion into our interpretation of the geological record. The first and fundamental point is to determine accurately the order of superposition of the strata. Until this is. done detailed palazontological classification may prove to be worth- less. But when once the succession of the rocks has been ﬁxed palmontological evidence may become paramount. From what has been above advanced it must be evident that, even if the several groups in a formation or system of rocks in any district or country have been minutely sub- divided by means of their characteristic fossils, and if, after