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The upper is more than five thousand feet thick and consists of shales and sandstones with some limestones. The lowest member is a basal conglomerate composed of pebbles derived from the erosion of the dark crumpled schists beneath, schists which are supposed to be Archean.
These diverse forms indicate that before the Algonkian had closed, life was abundant and had widely differentiated. We may expect that other forms will be discovered as the rocks are closely searched. PRE-CAMBRIAN GEOGRAPHY. Our knowledge is far too meager to warrant an attempt to draw the varying outlines of sea and land during the Archean and Algonkian eras.
THE LAKE SUPERIOR REGION. In eastern Canada an area of pre- Cambrian rocks, Archean and Algonkian, estimated at two million square miles, stretches from the Great Lakes and the St.
The archean age, during which the brooding seems to have gone on, was probably as long as all the ages since. How we are baffled when we talk about the beginning of anything in nature or in our own lives! In our experience there must be a first, but when did manhood begin; when did puberty, when did old age, begin? When did each stage of our mental growth begin?
|Quaternary |Recent Cenozoic | |Pleistocene | |Tertiary |Pliocene |Miocene |Eocene |Cretaceous Mesozoic |Jurassic |Triassic |Permian |Carboniferous |Pennsylvanian | |Mississippian Paleozoic |Devonian |Silurian |Ordovician |Cambrian Algonkian Archean
Thus where a region is underlain by sedimentary rocks, as is the drift-covered area from the Hudson River to the Missouri, the drift contains not only fragments of limestone, sandstone, and shale of local derivation, but also pebbles of many igneous and metamorphic rocks, such as granites, gneisses, schists, dike rocks, quartzites, and the quartz of mineral veins, whose nearest source is the Archean area of Canada and the states of our northern border.
At the present time the pre-Cambrian region about Lake Superior leads the world in the production of iron ore, its output for 1903 being more than five sevenths of the entire output of the whole United States, and exceeding that of any foreign country. A theory held by many refers the ultimate source of the iron to the igneous rocks of the Archean.
Great unconformities, UU' separate the Algonkian both from the Archean and from the Cambrian, and divide it into three distinct systems, the LOWER HURONIAN, the UPPER HURONIAN, and the KEWEENAWAN. The Lower and the Upper Huronian consist in the main of old sea muds and sands and limy oozes now changed to gneisses, schists, marbles, quartzites, slates, and other metamorphic rocks.
In some regions there rests unconformably on the Archean an immense body of stratified rocks, thousands and in places even scores of thousands of feet thick, known as the ALGONKIAN. Great unconformities divide it into well-defined systems, but as only the scantiest traces of fossils appear here and there among its strata, it is as yet impossible to correlate the formations of different regions and to give them names of more than local application.
The floor on which the Algonkian rests is remarkably even, and here again is proved an interval of incalculable length, during which an ancient land mass of Archean rocks was baseleveled before it received the cover of the sediments of the later age.
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