The scheme of classification of general application in North America is shown in the chart on page 308. The arrangement is in order of age, the oldest formation being at the bottom. There is some lack of uniformity among American geologists as to certain of the terms used, more especially in the lower portion of the column, and in part the scheme is provisional, but in general it may be taken as expressing the progress made in the study of the geology of North America up to the present time. The names of the larger divisions in this scheme of classification, or those designating the groups and systems and the eras and periods, have for the most part been adopted from European geologists. Two important ones, however--namely, Archean and Algonkian--are of American birth. While this scheme of classification is based on the succession of sedimentary beds, igneous and metamorphic rocks have a place in it, providing their age can be determined. The Archean period includes the time previous to the deposition of the oldest known sedimentary beds, and its lower limit is as yet undefined. The Archean system, or the rocks formed during the Archean period, are without known fossils, and consist largely of gneisses and foliated schists, which are metamorphosed sedimentary or igneous terranes, together with various eruptives. The typical area where these rocks are exposed at the surface is in the Laurentian Highlands of eastern Canada, the main Archean nucleus of the continent, but rocks of the same age and same general character occur in several of the mountain systems of both the Atlantic and Pacific cordilleras, and underlie the sedimentary beds throughout a large part of the Continental basin. The Archean system was named by J. D. Dana, and divided into two portions, namely, the Laurentian below and the Huronian above. More recent studies, especially by C. R. Van Hise, have shown the necessity of removing from the system many of the terranes formerly referred to it, and of placing them in the Algonkian. The Archean as it remains after this adjustment is termed by Van Hise the _Basement Complex_. This term, although thus far not generally adopted, has much to commend it, since the terranes designated by it are highly complex, and may perhaps be ultimately subdivided into two or more systems, and besides occupy a basal position lower than any known sedimentary formation that has escaped metamorphism. _Outline Chart of the Geological History of North America_