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620 - PALHONTOLOGICAL GEOLOGY.  [Boox V. 
But even where the record may originally have been tolerably full, 
powerful dislocations have often thrown considerable portions of it out 
of sight. Sometimes extensive metamorphism has so affected the 
rocks that their original characters, including their organic contents, 
have been destroyed. Oftenest of all, denudation has come into play, 
and vast masses of strata have been entirely worn away, as is shown 
not only by the erosion of existing land-surfaces but by the abundant 
unconformabilities in the structure of the earth’s crust. 
While the mere fact that one series of rocks lies unconformably 
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 
disturbed, upraised, denuded, and covered unconformably 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 
judgment as to the amount of interval, even measured by the vague 
relative standards of geological chronology. But if each group 
contains a well-preserved suite of organic remains, it may not only 
be possible, but easy, to say how much of the known 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 be paralleled with stage H. It would 
be then apparent that at this locality the chronicles of three great. 
geological periods H, F, and G were wanting, which are elsewhere 
found to be intercalated 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, or where the record 
of them has been preserved. 
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 abrupt and important 
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 striking paleontological — 
contrasts between unconformable strata are sufficiently explicable. 
It is not so easy to give a satisfactory account of those which oecur 
where the beds are strictly conformable, and where no evidence can 
