PROTEROZOIC (Greek proteros, early, and zoo life), a geologic time division generally given die rank of an era. According to the United States Geological Survey it includes all time before the Paleozoic, and is divided into the Archean and Algonkian periods. Other geologists divide the time before the Paleozoic into two eras, the Archeozoic (Archean) and Proterozoic (Algonkian). This article follows geological survey usage. Proterozoic time is also frequently referred to as the Pre-Cambrian. The type sections are in the region of the Great Lakes, in northern United States and Canada. The rocks of the era readily fall into two systems, the Archean (q.v.) and Algonkian (q.v.), separated by a profound unconformity. The former are dominantly igneous, consisting of a great series of basic lava flows and minor amounts of interbedded sediment, cut by large granite intrusions. This period of volcanism was followed by long erosion, and Algonkian rocks, dominantly sedimentary, were laid down on eroded Archean surfaces. Little is known
of the geography of either period, except that in Algonkian time considerable limestone was deposited, pointing to probable submergence under an ocean. It is believed that glaciers occupied parts of Canada in the Algon kian, and in the latter part of the period vol canism was again extensive.
There are no direct evidences of life in the Archean nor in the early Algonkian though limestones may point to the existence of mi nute lime-secreting animals. A few imperfect fossils, probably brachiopods and crustaceans, have been found in very late Algonkian rocks. Two similar rock systems, under various names, are recognized in Europe and probably in other continents. The intense metamorphism that Proterozoic rocks have undergone in many regions makes it impossible to separate the two systems over much of the world, at least until much more detailed work is done.