the abundance of plant life, but this supposition is not supported by any convincing evidence. The term Azoic[50] applied by some writers to these oldest rocks suggests the absence of life during the period in which they were formed. Life there must have been, though we are unable to discover its records. The period of time represented by the Archaean or Pre-Cambrian rocks must be enormous, and it was in that earliest era that the first links in the chain of life were forged. II. =Cambrian.= The term Cambrian was adopted by Sedgwick for a series of sedimentary rocks in North Wales (_Cambria_). In that district, in South Wales, the Longmynd Hills, the Malverns, in Scotland, and other regions there occur more or less highly folded and contorted beds of pebbly conglomerate, sandstones, shales and slates resting on the uneven surface of an Archaean foundation. It is in these Cambrian rocks that trustworthy records of organic life are first met with. Among the most constant and characteristic fossils of this period are the extinct and aberrant members of the crustacea, the trilobites; these with some brachiopods, sponges, and other fossils comprise the oldest fauna, of which the ancestral types have yet to be discovered. During the last few decades the number of Cambrian fossils has been considerably increased, and in certain regions of North America and China there are found many thousand feet of strata above the typical Archaean rocks and below the newer fossiliferous beds of Cambrian age. It is reasonable to suppose that future research may extend the present limits of fossil-bearing rocks below the horizon, which is marked by the occurrence of the widely distributed and oldest known trilobite, the genus _Olenellus_. The vast thickness of Cambrian strata was for the most part laid down on the floor of a comparatively deep sea; other members of the series represent the shingle beaches and coast deposits accumulated on the slopes of Archaean islands. There have been many conjectures as to the distribution of land and sea during the deposition of these rocks; but the data are too imperfect to enable us to restore with any degree of confidence the physical geography of this Palaeozoic epoch, of which the sediments stood out as islands of Cambrian land during many succeeding ages. III. =Ordovician.=