Throughout North America, geologists have long recognized in the great fundamental complex of rocks, known generally to-day as the Archean, a natural division into two well-characterized portions, related to each other in space as upper and lower. The lower division is commonly known as the Laurentian, and consists for the most part of an assemblage of rocks of the character of granites, syenites, diorites, and gabbros in mineralogical composition, but more or less foliated or gneissic. Involved with these in a way not hitherto understood there are also, in some regions, portions of various gneiss, schist, limestone, quartzite, and conglomerate formations, which, not being easily separable from the foliated granite rocks, have been sometimes classed with the latter as Laurentian.
Nomenclature.—The upper division is of very varied lithological character, and various names have been applied to it, or . . .