STRATIGRAPHIC CLASSIFICATION DIASTROPHIC CRITERIA 400 



ican continent, it is found that the displacement of the strandline is quite 

 different from that noted on the other sides. Here, as explained and illus- 

 trated on page 407, the movements of the first group caused gentle though 

 very widespread subsidence. In other words, while the results of the 

 movements are strictly orogenic on the east, south, and west borders of 

 the continent, on the northern side they resulted in sinking and gentle 

 warping of wide land areas, and thus are distinctly epeirogenic in char- 

 acter. When the subsidence proceeded to the submergent stage, as in the 

 late Black River, the earliest Trenton, and in one or more of the Rich- 

 mondian ages,^^ the northern sea transgressed very broadly and with ex- 

 traordinary rapidity over the great median flats of the continent. Proof 

 of the relative rapidity of these transgressions is seen in the small thick- 

 ness of the deposits of seas invading from the north, this being out of all 

 proportion to the great extent of territory covered by them. The seas in- 

 vading from the east, south, and west as a rule not only laid down much 

 greater thicknesses of sediment, but the inland extent of the deposits was 

 very much less, and the rate of sea advance, as indicated by the relative 

 abruptness of the inland overlaps of the successive beds, was much slower. 



From the foregoing it will be observed that the great north-south dif- 

 ferential movements referred to under the term "continental tilting" are 

 partly assignable to both groups. The northward tilt occasioned what 

 may be called the submergent phase of the total result of a movement of 

 the first group that manifested itself more typically in the south by crustal 

 folding and land building. Both phases suggest impulsive and on the 

 whole rapid development. The reversal of the tilt, occasioned by eleva- 

 tion of the northern and median parts of the continent and subsidence in 

 the southern, was a much slower process and therefore belongs to the 

 second group. 



All other tilting of continental areas, especially the "minor tilting'' of 

 such prevailingly positive inland domes as the Cincinnati, Nashville, and 

 Adirondack uplifts, which oscillated in east-west directions more com- 

 monly than north-southwardly, is similarly attributable to movements of 

 both groups. However, in these cases the reversal or decided modification 

 of the tilt of the domes is probably always more or less impulsive in 

 origin and consummation; hence due to a movement of the first group. 

 Apparently, then, only the gradual subsidences and consequent submer- 

 gences of one or another of their sides may be attributed to movements of 

 the second group. 



^ It should be admitted here that It is only when such northern invasions were accom- 

 plished that the diflPerential character of these movements is demonstrable on competent 

 stratlgraphic and faunal evidence. 



XXVIII— Bull. Geol. Soc. Am., Vol. 22, 1910 



