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It is about 100 years since geological work in the Canadian Shield began. The Geological Society of America has been in existence, therefore, during only half of this time. The work of the past 50 years, however, is so closely related to that of the previous 50 years that the two periods can scarcely be dissociated. Some introductory account of the investigations of the first half century in the Shield is, for this reason, necessary in attempting to outline the progress of geology in the last half.

Geological work in the Canadian pre-Cambrian Shield during the entire period i t has undergone investigation belongs in a general way to two classes: (1) preliminary or exploratory, and (2) economic. Geological maps made as part of investigations of the first class have been predominantly on the scale of 4 miles or more to 1 inch and practically all on scales less detailed than 1 mile to 1 inch. Maps of the second class, on the other hand, have been largely on a scale of 1 mile to 1 inch, or less commonly on more detailed scales. The scale on which geological maps are prepared is, therefore, an approximate criterion of the class to which the work of which i t is a part belongs, but in some cases, owing to the energy and concentration of the individual worker, the detail of the investigation far exceeds the scale of the map on which the results of the study are shown.

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