The calcareous Algae of the Middle Ordovician sequence of the St. Lawrence lowlands show a lateral and regional constancy which reflects the stability of the shelf at that time. In addition, the algal groups, when considered separately, suggest aptitudes to adapt themselves to various habitats and conditions.

Chazy rocks in the Lake Champlain area, New York, Trenton deposits outcropping in the Trenton region, New York, Black River section in its type section at Black River, New York, and the Simcoe Group in Lake Simcoe, Ontario, are studied and sampled for the examination of their algal contents.

Thirty-four taxa are identified in these various deposits. Petrographic evidences and the interpretation of the algal microfacies in the four regions studied reveal the presence of as many as 22 types of lithological units distinguishable on their algal content and their relation to specific paleoecological environments.

The abundance of Algae and algal components in the Middle Ordovician sequence, underlines the importance of their role in relation to sedimentation on the shelf in the regions studied. The diversity and the specificity of the Algae in the units and in the environments reflect a pattern of distribution which follows certain environmental controls similar to those prevailing in the modern seas.

The algal assemblages show responses to physical, biological, and chemical variations of the environments. These assemblages play in the sedimentation the same role assumed by recent Algae, production of carbonates, trapping and stabilization of the sediments, algal mats, formation of oncolites and algal encrustations, and edification of bindstones-framestones.

Although the Algae are not always useful as chronostratigraphic indices, they remain in the Ordovician successful paleoenvironmental indicators.

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