The geological ages

Abstract

The perspective of history begins with the origin of the earth, and develops through geological time until the stage is ultimately set for human evolution. The age of the earth, so long a matter of grave controversy, is now fairly reliably known as the result of the development of delicate methods of measurement that make use of the radioactive properties of certain naturally occurring elements, and is of the order of 4,500 million years. For much of this period there was apparently no life upon the earth, and certainly any life that existed left no traces that have yet been recognized as such. Yet year by year records of primitive life are announced from older and older rocks, until now there are claims going back more than 2,000 million years. Nevertheless, complex life forms that have left abundant traces as fossils are not found in rocks older than those of the Cambrian system, which may be accepted as having been laid down about 600 million years ago. This dividing line between Cambrian and Pre-Cambrian rocks, between those with abundant fossil remains and those to all intents and purposes without any is clearly of paramount importance to the palaeontologist studying the forms of organic evolution, and to the stratigraphical geologist who depends so importantly upon his labours. The greater part of all geological writing is thus concerned with Cambrian and Post-Cambrian time, but the non-geologist must be careful to avoid the inference from this that little of importance occurred before the Cambrian. More than four-fifths of the history of our earth was over before the fossil record opens.