THE GRENVILLE SUBPROVINCE forms an important part of the Precambrian Shield in eastern Canada. It covers an area 150 to 200 miles wide extending along the southern border of the Shield from Lake Huron to the Labrador coast and projects into the Adirondack region of the United States. This vast expanse of territory is distinguished from the remainder of the Shield by the presence of extensive areas of crystalline limestone intermingled with a variety of gneisses and highly metamorphosed sedimentary and volcanic rocks. Because of the great geological complexity and rock alteration the Grenville subprovince presents a variety of problems that have always puzzled and intrigued geologists.

A century of investigation has produced much factual information about the distribution of rock types, their relationships, alterations, structure, age, and valuable mineral content. This in turn has led to much speculation as to their mode of origin and relationship to adjacent Precambrian units. Although the Grenville subprovince has not been a prolific source of mineral production when compared with some other major subdivisions of the Shield, it contains the greatest variety of mineral species, and, in recent years, important deposits of iron ore, lead and zinc, uranium, and industrial minerals have been developed. This encouragement has greatly increased the incentive for mineral exploration throughout the whole region. The Grenville subprovince is now undergoing the greatest period of mineral search and development in its entire history.

With these facts in mind, the officers of Section IV (Geology and Allied Sciences) of the Royal Society of Canada decided that it was an opportune time to discuss all aspects of Grenville geology. Accordingly, a symposium on "The Grenville Problem" was arranged for the annual meeting of the Society at Toronto in June, 1955. Geologists with considerable experience in all aspects of Grenville geology were invited to contribute papers and take part in the discussions. They represented mining companies, government surveys, and universities. Thirteen papers were presented and discussed as fully as time would permit. It was felt that this information should be made available to a wider audience, and eventually, through the joint efforts of the Royal Society of Canada and the University of Toronto Press, several of the papers presented on that occasion were assembled in this publication. It is a sampling of areas and topics that are of current interest. It discusses a wide variety of subjects and questions that have come to the fore as geologists delve deeper into the mysteries of the Grenville rocks.

This is the first Canadian attempt to collect some of the available information and ideas on a difficult and fascinating subject. It is hoped, however, that it will serve a useful purpose and inspire further study and discussions on a geological unit that has a very considerable future potential in the Canadian mineral economy.

JAMES E. THOMSON is the Chairman of the Editorial Committee for The Royal Society of Canada Special Publications.

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