The Archæan (Pre-Cambrian) rocks of Britain have of late years received considerable attention, owing partly to the more or less complete working out of the younger groups, and partly, perhaps, to the fascination which attends a study of peculiar complexity. Whatever interest attaches to the correlation of formations which can be easily identified by their organic remains, or which can be traced across country for scores of miles in unbroken lines, it will be readily understood that the spirit of inquiry will be strongly piqued when it is challenged to construct orderly systems out of rock-masses to which the ordinary tests can be but partially applied. Yet much has been done towards the establishment of a succession amongst these ancient groups. In America, Dr. Sterry Hunt describes six distinct systems.