106 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 (Vol. II, chap. vii) 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 lighter 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 areseen 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 may even become obscured by 
extended peneplanation and marine transgression. 
In a subsequent paper,t 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 square 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 
t Bailey Willis, “A Theory of Continental Structure Applied to North America,” 
Bull. Geol. Soc. of America, Vol. XVIII, p. 392. 
