The Barrenness of Precambrian Rocks

Abstract

REFERRING to the paragraph in NATURE (February 28, p. 423), on the sudden appearance of a rich fauna in the Lower Cambrian rocks, I should like to make a suggestion for the consideration of geologists. May not the extreme poverty of organic remains in Precambrian (Archæan) strata be largely due to a scarcity of carbonate of lime in the water of the Precambrian seas? The Uriconian and Longmyndian rocks of Shropshire, which, at the very least, must include five miles of sediment, comprise hardly a scrap of limestone. The same remark will apply to the Precambrian strata of Charnwood, South Wales, the mainland of North Wales, and the great Torridonian group of Scotland. The Pebidian rocks of Anglesey contain bands of limestone, it is true, but it is highly probable that they are of chemical origin, and not derived from oceanic waters. There are, of course, plenty of limestones in the older Archæan rocks of North America, and a few of them in the Lower Archæans of Britain; but the proof of their marine origin remains to be written. They contain no undisputed organic remains. The rocks in which they are intercalated are not proved to be altered sedimentaries. There were numerous animals living in the Salopian area in the Longmyndian epoch, for their trails are quite abundant in some of the slaty seams; but, if there were no carbonate of lime in the sea, there could, of course, be little material to provide shells for its inhabitants. Numerous creatures of many types might have been evolved, whose soft tissue would leave no traces in the rocks. In succeeding ages, as the forces of denudation cleared off the newer Archæan, and cut down into partially decomposed crystallines, abundance of calcic carbonate would be carried down into the sea.