ARCILEAN TIME. 151 



In Europe, the Archaean system has been distinctly recognized in 

 northwestern Scotland ; in Finland, Norway, and Sweden ; Bohemia 

 (formations A and B of Barrande) ; Bavaria (Hercynian and Bojie 

 Gneiss). The great iron-regions of Sweden are of this age. 



II. Periods of the Archaean Era. 



In Canada, where these rocks in North America are most fully 

 represented, two periods have been recognized : 1, The Laurentian, 

 the older, so named from the river St. Lawrence ; and 2, the Huron- 

 ian. The estimated thickness of the rocks of the Laurentian period 

 is 30,000 feet ; of the Huronian, from 10,000 to 20,000 feet. 



1. LAURENTIAN PERIOD. 

 I. Rocks : Kinds and Distribution. 



Geographical Distribution. — The regions of Laurentian rocks com- 

 prise all the Archaean above mentioned, excepting the areas described 

 beyond as Huronian. 



A small part of the Canada Laurentian has been announced as probably unconform- 

 able on the rest; and Logan has suggested for it the name of the Upper Laurentian, or 

 Labrador beds. One area covers part of Montcalm and Terre-bonne ; another lies west 

 of Lake St. John ; others northeast of Montmorency Falls, and near St. Paul's Bay. 



Kinds of Rocks. — The rocks, with few exceptions, are metamorphio 

 or crystalline rocks. They include granite and gneiss and some mica 

 schist ; also, very prominently, rocks of the hornblende (and pyroxene) 

 series, as syenyte, hornblendic gneiss, and other kinds ; also extensive 

 beds of crystalline limestone. Besides these, there are quartzyte and 

 conglomerate. The lime-and-soda feldspar called labradorite — often 

 characterized by a beautiful play of colors — is common in Archaean 

 terranes, forming, with a lamellar mineral related to pyroxene or horn- 

 blende, the rock hypersthenyte. 



Chrysolite, a silicate of magnesia and iron, is a constituent of some 

 hypersthenyte. Apatite (phosphate of lime) is a common mineral, and 

 is often found in grains or crystals in the iron ores. 



Abundance of iron-bearing minerals is a striking characteristic of 

 the Archaean rocks. It is the cause of the frequent reddish color of 

 the feldspar of the granytic rocks. It is apparent in the prevalence of 

 rocks of the hornblendic series, the black variety of hornblende and 

 pyroxene, present in them, containing much iron. It is especially man- 

 ifested in the existence of immense beds of iron ore, which consist 

 either of magnetite (Fe 3 4 ), or of hematite (Fe 2 3 ) or of titanic 

 iron (the last differing from the others in having part of the iron re- 

 placed by titanium). The beds are occasionally one or more hundred 

 feet thick, as in the Missouri Iron Mountain, the Adirondack region 

 of New York, the Marquette region of the northern peninsula of 



