Calcareous algae, some of which are replaced by chert, are abundant in several pre-Cambrian, Cambrian, and Mississippian formations of the Northwest Territories of Canada and the Rocky Mountains of Wyoming and Montana, and in Ordovician formations of the upper Mississippi Valley. Several form-genera and form-species persist over considerable areas in spite of variations in the sediments which contain them. Some form-species produced massive biostromes, while others made limestone “mottlings” in dolomites or produced irregular layers. Fluctuations within form-species may be correlated with evidence of environmental variation.

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