Handbook of Paleontology 199 



has been altered to hematite and magnetite and is mined. 

 The Grenville series also is of widespread distribution, 

 but will be discussed separately below because of its 

 occurrence in New York State. The Laurentian gneisses 

 and granites have been touched upon. As they are intru- 

 sives they are younger than the Keewatin and Grenville 

 series into which they have been thrust. 



The Archeozoic was a time of unusual volcanic activity 

 and great deformation. Batholiths of granite are of so 

 frequent occurrence as to be almost characteristic of the 

 era, and in certain regions a large part of the rock sur- 

 face consists of them. They are broken, faulted and have 

 been intruded by lavas of later age. Dikes cut across the 

 schists and the Laurentian granites and gneisses. This 

 oldest series of rocks consists then largely of lava flows 

 and tuffs, with occasional beds of sedimentary rocks and 

 beds of iron, broken by faults and massive intrusives, 

 folded, contorted and so metamorphosed that former con- 

 ditions are obscured. Valuable deposits of minerals — 

 copper, iron, nickel, cobalt and silver — have promoted 

 the study of these rocks, detailed knowledge of which is 

 confined to the region around the Great Lakes and the St 

 Lawrence river where excellent exposures have been 

 revealed by glaciation. The Archeozoic era closed with 

 a period of mountain building, the Laurentian Revolu- 

 tion, at which time occurred the intrusion of the Lauren- 

 tian granites. This was followed by a long period of 

 erosion resulting in peneplanation of the highlands, the 

 Ep- Archeozoic Interval (epi, upon). 



The Proterozoic era represents at least one-fourth of 

 geologic time. It comprises in the Lake Superior region 

 53,000 feet of stratified rocks and 22,000 feet of vol- 

 canics ; in the Rocky Mountains area about 37,000 feet. 



