Page:The Journal of geology (IA journalofgeology11893univ).pdf/14

It is obvious that in any effort to establish that a group of rocks is older than the very base of the sedimentary fossiliferous formations, we must somewhere find that group emerging from under the bottom of these formations. Until lithological characters are ascertained to be so distinctive and constant as to be comparable to fossil evidence for purposes of stratigraphical identification, we should not assume that detached areas of older rocks rising amid Palæozoic, Secondary or Tertiary formations are pre-Cambrian. We should, if possible, begin at the bottom of the Palæozoic systems and work backward, tracing each successive system or group as these rise from under each other, until we arrive at what appears to be the oldest traceable within the region of observation. It is clear that in the present state of knowledge we have no satisfactory means of identifying such successive systems in widely separated countries. All that can be attempted in the meantime is to ascertain the special types in each region, and to point out their general resemblances or contrasts to those of other regions. It is better to avoid confusion by refraining from applying the stratigraphical names adopted for the oldest rocks of one region to those of another geographically remote, though we may hope that eventually it may be possible to work out the equivalence of these local names.

In the British Isles, by much the most important region for the study of the oldest rocks is to be found in the north-west Highlands of Scotland. The very basement strata of the Cambrian system are there traceable for a distance of more than 100 miles, reposing with a strong unconformability upon all rocks of older date. They consist of dolomitic shales with Olenellus, resting upon a thick group of quartzites, full of annelid tubes. One of the most remarkable features of these ancient strata is the persistence of their component bands or zones which, though sometimes only a few feet thick, can be traced throughout the whole tract of country just referred to. For the study of the pre-Cambrian rocks this is an important point, for we can be quite certain that even where fossil evidence locally fails, the