The Cambrian System by definition must include the rocks (in the Caledonian geosyncline of Wales) to which this name was first applied. Its type sequence, however, does not define its boundaries. Evidence on the base of the Cambrian can be found in the Caledonian sedimentary areas of Scandinavia and in the adjoining Baltic region where the Blue Clay contains the oldest known Cambrian fauna. Further east, older rocks of similar character are known and there are references to glauconites over 1,200 million years (m.y.) old in these rocks. The base of the Cambrian cannot be extended down to the base of the sedimentary cover of the shelves surrounding the old shields. “Eocambrian” is probably the oldest defined term suitable for late Precambrian rocks which often resemble the Cambrian lithologically. They may contain a similar microflora, though their fauna which is rare and consists mostly of soft‐bodied animals, is different. There are in many areas tillites in rocks which could be Eocambrian, but their correlation is not yet definitely established. The definition of the base of the Cambrian rests therefore on biostratigraphic evidence and on chronological data (Glaessner, 1963).