ARCHJ3AN TIME. 



449 



real conglomerate," giving evidence of "attrition," "fragmental accumula- 

 tion," and subsequent metamorphism. The rounded stones were four to five 

 inches through, and consisted of crystalline augitic and other rocks. 



In the recognized Huronian areas on the north shore of Lake Huron, 

 and in the Penokee-Marquette belt, south of Lake Superior, extending 

 from Wisconsin into northern Michigan, the rocks are quartzyte, siliceous 

 schist, sandstones, conglomerates, micaceous and chloritic slates, chloritic 

 greenstone, dioryte ; and in Wisconsin there is a cherty limestone at the 

 base, and carbonaceous as well as graphitic shales above. 



A common feature of Archaean rocks, or at least of their veins, is the frequent occurrence 

 of minerals containing rare elements, as niobium, tantalum, lanthanum, thorium, yttrium, 

 zirconium, caesium, rubidium, and others. The following minerals ai-e common in 

 Archaean rocks, or their veins: nephelite (elseolite), cancrinite, sodalite, spinel, chryso- 

 heryl, danburite, amblygonite, spodumene, petalite, microlite, gadolinite, cryolite, besides 

 others. But garnet, mica, andalusite, cyanite, staurolite, are less common than in later 

 crystalline rocks. Chondrodite is usually, if not always, Archaean. 



In the Kent-Cornwall ridge, west of Kent, Conn., and in the high land east of 

 Tyringham, Lee, and Pittsfield, Mass., occur chondroditic limestones, "like that of Sussex 

 County, N.J., and at a locality east of South Lee, near the junction of the Archaean 

 rocks with the Cambrian quartzyte, masses of chondrodite occur as large as the fist. 



One of the most characteristic features of the Archaean is the occurrence 

 of great beds of valuable iron ore, some of them 100 to 400 feet thick. They 

 axe found of great thickness in Canada, northern and southeastern New- 

 York, northern Kew Jersey, and the region south through Virginia to 

 Georgia ; in the Penokee-Marquette belt, south of Lake Superior ; the 

 Missouri Iron Mountain region; also in Utah, Wyoming, Colorado, New 

 Mexico, and Arizona, and elsewhere. 



The ores are usually magnetite, hematite, and titanic iron, of bright, 

 lustrous kinds ; and in one region, in Sussex County, N. J., it is a zinc- 

 manganese iron ore, called franklinite, mixed with disseminated zinc oxide 



497. 



Northern Michigan, Whitney. 



498. 



Essex County, N.T. 

 Emmons. 



499. 



Essex County, N.T. 

 Emmons. 



and zinc silicate. But, besides these kinds, there is also iron carbonate or 

 siderite. 



Figs. 497 to 499 show some of the positions of the ore-beds in 

 metamorphic schists, the black beds i being the ore-beds, and the ore 

 magnetite or hematite. 



In Pig. 497, the ore-beds (of northern Michigan) are between beds of 



DANA'S MANUAL — 29 



