North America . er portion of the column, and in partthe scheme is provisional, but in general it may be takenas expressing the progress made in the study of the geol-ogy of North America up to the present time. The names of the larger divisions in this scheme ofclassification, or those designating the groups and systemsand the eras and periods, have for the most part beenadopted from European geologists. Two important ones,however—namely, Archean and Algonkian—are of Ameri-can birth. While this scheme of classification is based on thesuccession of sedimentary beds, igneous and metamorphicrock
North America . er portion of the column, and in partthe scheme is provisional, but in general it may be takenas expressing the progress made in the study of the geol-ogy of North America up to the present time. The names of the larger divisions in this scheme ofclassification, or those designating the groups and systemsand the eras and periods, have for the most part beenadopted from European geologists. Two important ones,however—namely, Archean and Algonkian—are of Ameri-can birth. While this scheme of classification is based on thesuccession of sedimentary beds, igneous and metamorphicrocks have a place in it, providing their age can be deter-mined. The Archean period includes the time previous to thedeposition of the oldest known sedimentary beds, and itslower limit is as yet undefined. The Archean system, orthe rocks formed during the Archean period, are withoutknown fossils, and consist largely of gneisses and foliated GEOLOGY >09 Outline Chart of the Geological History of North America. schists, which are metamorphosed sedimentary or igneousterranes, together with various eruptives. The typicalarea where these rocks are exposed at the surface is in theLaurentian Highlands of eastern Canada, the main Archeannucleus of the continent, but rocks of the same age andsame general character occur in several of the mountainsystems of both the Atlantic and Pacific cordilleras, andunderlie the sedimentary beds throughoul a large pari ofthe Cuitmental basin. The Archean system was namedby f. I). Nana, and divided into two portions, namely, the 21 310 NORTH AMERICA Laurentian below and the Huronian above. More recentstudies, especially by C. R. Van Hise. have shown the neces-sity of removing from the system many of the terranes for-merly referred to it, and of placing them in the Archean as it remains after this adjustment is termedby Van Hise the Basement Complex. This term, althoughthus far not generally adopted, has much to commend it,since the te
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