Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/332

Rh 318 X. L'.'co.'romumL1rr. Where one series of rocks, whether of aqueous or igneous origin, has been laid down continuously and without dis- turbance upon another series, they are said to be conform- able. Thus in ﬁg. :39 the sheets of rock numbered 1, 2, 3, .— .1 A Fm. 59.—0verlnp of conformable strata. and 4 have succeeded each other in regular order, and ex- hibit a perfect conformability. They overlap each other, however, X0. 2 extending beyond the edge of No. 1, No. 3 beyond that of No. 2?, and so on. As already explained (p. 295), this structure points to a gradual subsidence and enlargement of the area of deposit. But all these conform- able beds repose against the older platform A, with which they have no direct connexion. That plat-form may consist of horizontal or inclined strata, or contorted schist, or crystalline igneous rocks. In any case there is a complete break between it and the overlying rocks, which rest successively on different parts of the older mass. This relation is termed an uuconformability. The upper conform- able beds in ﬁg. 59 are said to lie unconformably upon A. It is evident that this structure may occur in ordinary stratiﬁed, or in igneous, or in metamorphic rocks, or between any two of these great series. It is most familiarly dis- played among stratified masses, and can there be most satis- factorily studied. The lines of bedding furnish a ready means of detecting differences of inclination and discordance of superposition. But even among igneous protrusions and in ancient metamorphic masses, distinct evidence of uncon- formability is not always difficult to trace. Though conformable rocks may usually be presumed to have followed each other continuously without any great disturbance of geographical conditions, we cannot always be safe in such an inference. But an unconformability leaves no room to doubt that it marks a decided break in the con- tinuity of deposit. Hence no kind of geological structure is of higher importance in the interpretation of the history of the stratiﬁed formations of a country. In rare cases an unconformability may occur between two horizontal groups of strata. In ﬁg. 60, for instance, a set of beds C is shown Fm. 60.-—L'nconI'o11m.bility among horizontal beds. to lie conformably for some distance upon an older series (I. Were nothing more to be seen than what appears towards the right hand, we might justifiably conclude the whole of the rocks to be conformable. By passing to the left, how- ever, we should find evidence of the older group having been upraised and unequally denuded before the deposition of the newer. The denudation would show that the conformability was accidental, that the older rocks had really been upraised and worn down before the formation of the newer. In such a case the upheaval must have been so equable as not to disturb the horizontality of the lower rocks. As a rule, however, it seldom happens that movements of this kind have taken place over an extensive area so (2 EOLOG Y [1v. STRUCTURAL. equably as not to produce a want of conformability some- where between the older and newer rocks. ;lost frequently the older formations have been disturbed, tilted at various angles, or even placed on end. They have likewise been irregularly and enormously worn down. Hence, instead of lying parallel, the younger beds run transgressively across the upturned denuded ends of the older. The greater the 1} (I I-‘ic. 6l.—Scction of unconformablc rocks. disturbance of the older rocks the more marked is the un- conformability. In ﬁg. 61, for instance, the series of bed.- A is uncomformably covered by the series C. At both sides of the arch the uncomformability is strongly marked, but at the centre (l the two series seem to be conformable. An unconformability forms one of the great breaks in the geological record. In the foregoing ﬁgure, by way of illus- tration, we see at once that a notable hiatus in deposition, and therefore in geological chronology, must exist between series A and C. The older rocks had been deposited, folded, up- heaved, and worn down before the accumulation of the newer series upon their denuded edges. These changes must have demanded a considerable lapse of time. Yet, looking merely at the structure in itself, we have evidently no means of ﬁxing, even relatively, the length of interval marked by an unconformability. The mere violence of con- trast between a set of vertical beds below and a horizontal group above it is no reliable criterion of the relative lapse of time between their deposition, for an older portion of a given formation might be tilted on end aml be overlaid un- conformably by a later part of the same formation. A set of ﬂat rocks of high geological antiquity might, on the other hand, be covered by a formation of comparatively recent date, yet in spite of the want of discordance between the two, they might have been separated by a large portion of the total sum of geological time. It is by the evidence of organic remains that the relative importance of unconform- abilities must be measured, as will be explained in part v. Paramount though the effect of an unconformability may be in the geological structure of a country, it must never- theless be in almost all cases local. The disturbance by which it was produced can have effected but a comparatively circumscribed region, beyond the limits of which the con- tinuity of sedimentation may have been undisturbed. We may therefore always expect to be able to ﬁll up the gaps in one district from the more complete geological formations of another. In ﬁg. 61 we see that something is wanting between A and C. But in the structure of another country or a different part of the same country we might discover /4

.___.____g__.._-.- Fic. 62.——Section showing the groups of fig. 61 in confmmablc sequence,“-itln the intervening blank (B) supplied. the complete succession, as in fig. 6'2, where the whole of the rocks succeed each other conformably, and where the gap between A and C marked by the unconformability in ﬁg. 61 is bridged over by the intermediate group of strata B.