lo FRANK D. ADAMS 



subject which I hope may at least be suggestive of a line along which 

 some advance may be made in the correlation of these ancient rocks. 



In his Research in China (\'ol. II, chap, viii) Dr. Bailey Willis 

 has put forward a theory to account for the origin of continental 

 structure. In each of our present continents there are areas which 

 during the evolution of the continent have always tended to rise — 

 these he calls positive elements. There are certain other areas which 

 have always shown a tendency to sink, relatively to the adjacent 

 masses — these he calls negative elements. The movement of these 

 elements is due to the greater relative density of the negative elements 

 causing them to sink, while the relatively hghter positive elements 

 tend to rise so as to bring about an isostatic adjustment. There 

 have been horizontal movements as well as those in a vertical direction. 

 These are of notable magnitude and their effects are seen in the 

 schistose structure of these once deep seated rocks and the overthrust 

 and folded structures of the more superficial strata. The tendency 

 toward vertical displacement has actually resulted in movement only 

 at long intervals and during relatively short periods. Hence we may 

 recognize cycles of diastrophism each one of which comprises (a) a 

 comparatively brief period of orogenic and epeirogenic activity which 

 results in elevated lands and restricted mediterranea; and (b) a 

 comparatively long period of continental stability, which results in 

 extensive peneplanation. The critical times which bring out con- 

 tinental structure are the epochs of diastrophic activity. During 

 periods of inactivity the distinction between the positive and negative 

 elements becomes less obvious and mav even become obscured bv 

 extended peneplanation and marine transgression. 



In a subsequent paper,' the same writer outlines the positive and 

 negative elements of the continent of North America. The Canadian 

 Shield, which is also called Laurentia, is at once the largest and the 

 most readily distinguished positive element of the continent. It has 

 an area of approximately two million scjuare miles and the true bound- 

 ary may be traced along the St. Lawrence Valley into the deep of 

 Baffin's Bay and then north of the Arctic Archipelago (which is 

 scarcely to be separated from Greenland) across the Arctic Ocean 



I Bailey Willis, "A Theory of Continental Structure Applied to North America," 

 Bull. Ceol. Soc. oj America, Vol. XVIII, p. 392. 



