On the Predevonian Rocks of Bohemia

Abstract

Summary In very many of the Predevonian rocks of Bohemia we find marked lithological resemblances to the corresponding horizons in Britain. The Precambrian series is divisible into two distinct groups, similar to the Dimetian and Pebidian groups of Britain, and lying unconformably to one another and to the overlying Cambrian rocks. The beginning of the Cambrian period is marked, as in Britain, by a coarse conglomerate, which is probably of later formation than that of our own island, although it is probably homotaxeous with it. Moreover, there are strong resemblances between the various bands of the Bohemian and British Cambrian beds, especially marked in the deeper water deposits. The palæontological similarities are in many cases not so striking as the lithological. In the Silurian period, the resemblances, both lithological and palæontological, between Bohemia and Britain were less striking than in the Cambrian. The limestones so abundant in the Bohemian series seem to be lenticular masses having no wide extension; the most regular limestone is that of E 2, and this is about the horizon of, and comparable with, the Wenlock Limestone. But although the beds of this period as a whole are not so strictly comparable in the two countries as those of the Cambrian, we nevertheless find most remarkable coincidences between the zones of the lowest band, i. e. the Graptolite- bearing shales. The igneous rocks do not present many unusual features; but the occurrence of mica-traps in the Cambrian beds is very noteworthy, limited as they are, so Objection may be raised to my using Prof. Sedgwick’s nomenclature for the rocks of a region which M. Barrandehas so conclusively shown to possess three well-marked faunas. But the foregoing observations seem to show that. the break between the upper and middle faunas is much more marked than that between the middle and lower. In the first place, the deposition of the Cambrian beds does not seem to have begLm over the whole area until thG close of the Primordial era, the rocks containing the Primordial fauna, as well as the underlying conglomeratic series, being only locally developed, whilst it is not until the overlying beds of d 1 a that any thing like continuous deposition appears to have set in. Again, if I am right in referring d 1 α & β to the Lingula Flags and Tremadoc Slates respectively, the unconformity between the Primordial zone and second fauna of Bohemia does not occur at the same horizon as that between the Primordial zone and overlying beds of Britain, for the equivalents of d 1 α & β are included in the Primordial beds of Britain, or Cambrian of Sir C. Lyell, Dr. Hicks, and Mr. Lapworth. The break between the second and third faunas of Bohemia, on the other hand, occurs at the same horizon as in Britain, viz. between the top of the Bala beds and the Graptolitic fauna of Birkhill age. Lastly, the beds of d 1 α & β have as yet yielded by no means a satisfactory fauna, so that we do not here get one fauna in beds immediately succeeding another, as is the case with the beds at the top of the second and base of the third faunas of M. Barrande. But the connecting link between these two latter periods was supposed to have been discovered in the colonies of M. Barrande. I feel justified in saying that I have brought forward sufficient evidence to warrant a reconsideration of the data on which the theory of colonies was founded, and to give some grounds for the supposition that they are only portions of the band e 1 faulted down among the grits and shales at the summit of the Cambrian series.