ARCHiEAN TIME. 445 



SUBDIVISIONS OF THE ARCH^AN TERRANES, AND THE ROCKS. 



Subdivisions. — Two subdivisions have general acceptance : — 



I. The Laurextian. — Logan, Rep. Geol. Canada, for 1852-53; named 

 from the Laurentide Mountains. 



II. The Huronian Era. — Huronian of Logan and Murray, Rej). Geol. 

 Can., for 1853-4-5, in the special report for 1854 ; Esquisse du Geol. du Can., 

 1855. " Huron Cupriferous Formation " of the north shore of Lake Huron, 

 Rep. Geol. Can., for 1847-8. — Part of Agnotozoic, Irving, 1887, the Keweenaw 

 group of the Agnotozoic being referred beyond to the Paleozoic. Part of 

 Algonkian, "Walcott, 1889 ; a name proposed as a substitute for Agnotozoic, 

 and so accepted by geologists. 



The subdivisions were based, according to Logan, on relations of uncon- 

 formity in bedding between the Huronian and Laurentian terranes. The 

 Huronian areas recognized were situated along the north shore of Lake 

 Huron, and at points on the north and east shores of Lake Superior. 

 Archaean rocks vary from massive crystalline kinds, like granite, syenyte, 

 dioryte, and massive gneisses, to the thinnest of schists ; and include, also, 

 limestone, quartzyte, and some uncrystalline sandstone and other fragmental 

 beds, besides large beds of iron ore. The Laurentian division in the vicinity of 

 the lakes was observed to comprise the more massive kinds ; and the Huronian, 

 the thinner schists, as mica schist, chlorite schist, with quartzyte. With this 

 distinction in view, the Huronian was made to include also an area south 

 of Lake Superior extending from Marquette, Mich., westward, containing 

 the large beds of iron ore of that region; and this conclusion has since 

 been sustained by evidence proving their unconformability to the Archaean 

 terranes beneath. But most other references of areas to the Huronian that 

 have been made are reasonably questioned, because it is now known, as stated 

 on page 458, that the distinction based on kinds of rocks is not a safe cri- 

 terion of geological age. Among metamorphic Paleozoic rocks, massive, 

 thick-bedded and thin-bedded schists are associated in the same formation ; 

 and so it is, beyond doubt, in the Huronian, and even in the Laurentian. Still, 

 the thinner schists of the Archaean are to a much larger extent Huronian 

 than Laurentian ; and all the uncrystalline Archaean strata are Huronian. 



The beds of iron ore have so great thickness in some regions, that the 

 Archaean has been called the Iron Age in the earth's history. 



The localities of Huronian described by Logan with special detail in the Canadian 

 Geological Report of 186-3 are as follows: (1) to the west of the Mississaga River, north 

 of Lake Huron ; (2) to the eastward, in the vicinity of White Fish and Sturgeon rivers ; 

 (3) near Lake Temiscaming, 150 miles northeast of the last locality ; and a few miles from 

 Michipicoten Island, north of Lake Superior. The iron-bearing rocks south of Lake 

 Superior about Marquette and to the westward are referred to the same period on the 

 colored map in the octavo Atlas accompanying the Report, published in 1863, after inves- 

 tigations by Murray. 



