in Eastern North America. 401 



ther subdivided diastrophism into orogeny or mountain- 

 making and epeirogeny or continent-making. 8 Orogenic 

 movements are displayed in the narrower waves of uplift 

 in the terrestrial crust, and are associated with the more 

 energetic manifestations of diastrophism, while the epeir- 

 ogenic, so far as known to us, are rather displayed in slow 

 secular deformation of the crust." 



The term epeirogenic signifies continent-making, i. e., 

 the broad vertical movements within the continental 

 masses. Therefore to keep our ideas of the epeirogenic 

 changes, measured from the sea-level, which in the main 

 are upward or positive, distinct from the broad move- 

 ments of the oceanic basins, which as a rule are downward 

 or negative, we propose to refer to the latter as ocean- 

 ogenic. "We would also include under oceanogenic results 

 the consequent eu static changes in the level of the marine 

 waters. "We should be careful in using these terms, how- 

 ever, to note the fact that we do so only in a relative 

 sense, i. e., relative to the strand-line of the oceans, the 

 plane for measurements of the heaving or sinking masses, 

 because, since the earth is a shrinking mass, and the con- 

 tinents are accordingly to be looked upon as horsts, the 

 sum of the movements of both the continental masses and 

 the oceanic areas is downward. Nevertheless, under this 

 hypothesis the denser oceanic sectors shrink the most, 

 while the lighter continents as a whole appear to be 

 moved outward or upward, but in reality stand still as 

 horsts. It is therefore the eustatic or worldwide move- 

 ments of the variable oceanic level that give us the datum- 

 plane from which we measure the amount of continental 

 protuberance. 



We would then define orogeny as the resultant of the 

 lateral movements of the lithosphere, producing marked 

 foldings or f aultings, or both, that may be accompanied by 

 igneous intrusions. The orogenies are believed to be due 

 to earth shrinkage setting up in the lithosphere long- 

 enduring accumulations of strains that eventually attain 

 the breaking point, finding relief in orogenic epochs 

 when folding and faulting take place at the surface. 

 Periodically recurring epochs of orogeny are coming 



8 The Century Dictionary writes these terms epirogeny and epirogenic, 

 and defines them as "pertaining to movements by which changes of level 

 have occurred without special orogenic results; noting continent-making 

 movements or the up-and-down movements or oscillations of continental 

 areas. ' ' 



