In the Grand Canon and Montana determinable fossils have been found in the less changed sedimerits, but they are too few and scanty to tell us much of the life of the times. Evidences of life are not wanting in the metamor-phic rocks of the eastern and northern regions, but they are indirect. The strata of crystallized limestone are indications of the presence of animal life in the Algonkian seas. The great quantities of graphite diffused through many of the schists, and the beds of iron ore likewise tend to show the existence of plants at the same time. More conclusive are the determinable fossils obtained in the Belt series of Montana and in the Grand Canon series, which include the tracks of worms, brachiopods, and fragments of large Crustacea referable to the Eurypterida. Such remains imply a long antecedent history of life, the records of which remain to be discovered.
The pre-Cambrian rocks are remarkable for their wealth of valuable minerals. Immense accumulations of iron ore occur in Canada, New York, New Jersey, along the Appalachians from Virginia to Georgia, in Michigan, the Lake Superior region, Missouri, and the Southwest. The great copper mines of Lake Superior are associated with igneous rocks which intersect sandstones referred to the Algonkian.
It will be obvious to the student how very little is really known regarding the most ancient rocks of the earth's crust, the Archaean. They are enormously thick metamorphic masses of vast geographical extent. In all the continents they form the foundation upon which the oldest fossiliferous sediments were laid down, and, in brief, they are the oldest, the thickest, the most widely distributed and the most important of all the accessible constituents of the earth's crust. Their uniform character, wherever found, the extreme plication and metamorphism which they have undergone, and their world-wide distribution, are all extremely remarkable features, such as recur in rocks of no other age. The Algonkian sedimentary rocks present the earliest chapters in the recorded history of life. The pre-Cambrian rocks indicate that vast periods of time had elapsed before the clearly recorded part of the earth's history began, a time probably longer than all subsequent periods.