UNCONFORMABLE STRATA are strata which rest on the more or less inclined edges of older beds. The existence of unconformability in a series of strata is an indication of an interval sufficiently long to permit of the consolidation, disturbance, and upheaval, denudation, and subsequent depression of the inferior beds. No indication of the per iod that has intervened is to be found in the uneonformability itself; but some idea of it may be obtained by an examination of the strata that are known to have been depos ited subsequent to the inferior rocks, and previous to the overlying unconformable deposits. Thus,.in the n. of Annandale, the Silurian basement rocks, which have often an almost perpendicular dip, are covered by permian sandstone, and this, again, by the bowider-clay, or alluvial deposits. The first break in the strata represents the time dur ing which the Devonian and carboniferous rocks were deposited, when, in all probabil ity, the Silurian strata formed a thy land surface, and supplied some of the materials for these rocks. The second break is all the indication in that district of the lengthened period'dnring which the whole of the secondary and tertiary strata were hieing deposited here. The temporal value of the break is not so easily determined, in the majority of cases. It is only iu one place in Britain, in a cutting of the St. Helen's railway near ormskirk, where any apparent unconformability exists between the Bunter and Kenper strata, and even there it is so slight that it was long overlooked; yet this break repre sents a gap whieh on the continent is filled by the important sets of strata, the musehel katk and St. Cassian beds. containing two great assemblages of fossils perfectly distinct
from each other. Very frequently, however, no beds tire known which fill up the gap between the two uncoil formable series. Prof. Ramsay has shown that in the paleozoic epoch between the Laurentian gneiss and the permian beds there are ten breaks. Each of these is accompanied by a sudden and remarkable change of fossils, sometimes in the genera. and always in the species. Prof. Ramsay believes these gaps represent at much grea1a interval of time than that to which all the existing paleozoic formations of Great Britain bear witness. Such blanks in the stony records of the world's history are as frequent in the secondary and tertiary epochs as in the paleozoic.
The not taking into account the existence of unconformable stratification, has fre quently caused a useless expenditure of money in searching for minerals. It seemed natural to expect that the permian rocks of upper Annandale covered beds of the true coal-measures, but an examination of the numerous natural sections where the base of the permiau sandstone is seen, shows that it rests on the Silurian rocks; and the neces sarily abortive attempts that have been made to reach coal through the red sandstone have been simply a useless throwing away of money.