Since the War there has been a great increase in work devoted to determining the structure of the Canadian Shield. In this paper recent studies by nine geophysical methods are described and placed in their proper geological setting.
Some of these methods, especially those employing gravimeters, airborne magnetometers and radioactive age determinations are already in widespread use. Of others much less is known. These include seismic and thermal measurements upon the shield, studies of the distribution of radioactive elements and of isotope ratios of elements, measurements of temperatures and pressures at which minerals were deposited from which depths of erosion may be deduced. A brief outline of some mathematical theories concerning the mechanism of Earth failure has been included.
The paper is primarily devoted to describing methods, not results, but it is mentioned that the present evidence is compatible with the view that the continent has grown by the accretion of roots of marginal mountain ranges. It is suggested that a better understanding of the nature of the shield can be expected soon when the new methods have been more widely applied and when the results have been more fully tested and published.