(Geol.) Pertaining to or designating a period or era recognized by the United States Geological Survey and some other authorities, between the Archaean and the Paleozoic, from both of which it is generally separated in the record by unconformities. Algonkian rocks are both sedimentary and igneous. Although fossils are rare, life certainly existed in this period.
... immense area east of the main divide, extending from the middle of Montana up to the Yellowhead Pass in Alberta, or over 350 miles long, where the tops of the mountains consist of jointed limestones or argillites of Algonkian or pre-Cambrian "age," resting on soft Cretaceous shales. Often the greater part of the mass of a range will consist of these "older" and harder rocks, which by the erosion of the soft underlying shales are left standing in picturesque, rectangular, ... — Q. E. D., or New Light on the Doctrine of Creation • George McCready Price
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