The Journal of Geology, January-February 1893 / A Semi-Quarterly Magazine of Geology and Related Sciences

Table of Contents
JANUARY-FEBRUARY, 1893.
During the last twenty years much has been written about the pre-Cambrian rocks of the British Isles. Unfortunately when attention began to be sedulously given to the study of these ancient formations, the problems of metamorphism were still a hundred fold more obscure than they have since become; the aid of the microscope had not been seriously and systematically adopted for the investigation of the crystalline schists, and geologists generally were still under the belief that the broad structure of these schists could be treated like those of the sedimentary rocks, and be determined by rapid traverses of the ground. We have now painfully discovered that these older methods of observation were extremely crude, and that the work performed in accordance with them is now of little interest or value save as a historical warning to future generations of geologists. Geological literature has meanwhile been burdened with numerous contributions which remain as a permanent incubus on our library shelves.
It may serve a useful purpose at the present time in possibly aiding those who are engaged in the study of the oldest rocks of North America, if I place before them, as briefly as possible, the main facts which in my opinion have now been satisfactorily proved regarding the corresponding rocks of Britain, and if I indicate at the same time some of the more probable inferences in those cases where the facts, at present known, do not warrant a definite conclusion.

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.