364 PRE-CAMBRIAN PERIODS 



the rocks in which they occur may eventually prove to be Cam- 

 brian. Evidences of life are not wanting in the metamorphic 

 rocks of the eastern and northern regions, but they are indirect 

 and not entirely conclusive. The strata of crystallized limestone 

 are indications of the presence of animal life in the Algonkian 

 seas, for the only way in which such masses of limestone can be 

 formed now is by the organic agencies (see Chapter IX). 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. These indications do not amount to a 

 proof of the presence of life, for it is possible that the limestone, 

 graphite, and iron accumulations were made by chemical processes. 

 Radiolaria have been reported from some of the pre-Cambrian 

 schists of France (this has lately been questioned). 



The pre-Cambrian crystalline rocks are remarkable for their 

 wealth of valuable minerals. Immense accumulations of iron ore, 

 in beds from 100 to 400 feet thick, occur in Canada, New York, 

 New Jersey, along the Appalachians from Virginia to Georgia, 

 in Michigan, the Lake Superior region, Missouri, and the South- 

 west. The great copper mines of Lake Superior are in igneous 

 dikes 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. 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 and meta- 

 morphic rocks seem to represent the first series of deposits made 

 under water and the earliest chapters in the 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 taken together. 



