In western Quebec and adjacent parts of northeastern Ontario, about 2600 square miles underlain by Archean rocks has been remapped in recent years mainly on the scale of 1000 feet to 1 inch. This work is of special interest not merely because the territory mapped includes three of the most important mining camps in Canada, but because it is by far the largest area of Early Precambrian rocks in the Canadian Shield in which the detailed structural succession of formations has been determined with reasonable certainty. This paper is partly a short description of the lithological character, structure, and relationships of the bedrock formations of this region, and partly a consideration of the problems of nomenclature, correlation, and origin that the information afforded by this new work presents.

The greater part of the Early Precambrian rocks of the area remapped are in three east-trending belts:to the north, a highly folded, widespread, conformable, volcanic succession, usually called Keewatin but named Abitibi Series by the writer; to the south, the siliceous mica schist with some interbedded volcanic rocks of the Pontiac group occurring in a zone about 10 miles wide and 100 miles long; and between these a synclinorium of Timiskaming conglomerate and graywacke more than 125 miles long. North of the rocks of the Abitibi Series, however, a zone of sediments believed to be partly Timiskaming and partly pre-Timiskaming has been traced discontinuously by remapping from Destor township in Quebec to the Porcupine mining camp in Ontario. Although the last-mentioned sedimentary rocks lie outside the Timiskaming drainage basin, they are in territory usually regarded as part of the Timiskaming region. Their relationships are therefore described, and their correlation briefly discussed.

In the Timiskaming region: (1) a great structural unconformity separates the Timiskaming sediments from the rocks of the Abitibi series; (2) the Timiskaming rests unconformably on the Pontiac group; (3) an enormous thickness of coarse, poorly sorted, heterogeneous conglomerate occurs at the base of the south limb of the Timiskaming synclinorium in the Rouyn-Beauchastel area of western Quebec and in McElroy township in Ontario; (4) thin beds of laminated graywacke are interstratified with the conglomerate of the Timiskaming series even where it contains boulders up to 5 feet in diameter; and (5) the evidence indicates that the granitic rocks of the belt of banded gneisses separating the Grenville from the Timiskaming subprovince intrude the sediments of the Grenville Series, the Pontiac group, and the Timiskaming Series, but are overlain with great unconformity by the Huronian Cobalt Series.

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