502 C. SCHUCHERT CHRONOLOGY ON BASIS OF l'ALEOGEOGRAPHY 



assemblage, or, in other words, the distribution of the sedimentary and 

 faunal episode. Primary reliance is, of course, placed on the radiation 

 of the fauna mapped, while the shorelines are determined from the nature 

 of the deposits and the strike of the geosynclines. 



Of considerable value in deciphering the probable extent of the trans- 

 gressions whose traces are subsequently covered by other deposits is the 

 geographic pattern of earlier and later invasions — that is, the situation 

 of the positive and subpositive continental elements which remain fairly 

 constant. An analysis of the American Paleozoic formations shows that 

 they occur in greatest number and best development in the periodically 

 sinking subpositive or geosynclinal areas, and less continuously in the 

 neutral regions, and that these shallow seas are situated between rising 

 positive regions where there is renewal of previous records and the fur- 

 nishing of sediments. Along the Pacific border of the United States 

 deposition may have all been restricted to the geosynclines, but in Canada 

 and Alaska, previous to the Sierra Xevada uplift, there were, in addition, 

 wide neutral areas with short sedimentary cycles. 



All pioneer work in paleogeography must be very imperfect, and the 

 attempts for the Pacific border will long remain the least satisfactory, 

 because so little is as yet known of the geology of this difficult and not 

 easily accessible portion of North America. My maps probably err most 

 in that the marine transgressions depicted are too small in area, but it 

 will be easy to enlarge them when the evidence is at hand. On the other 

 hand, T strongly urge others to portray the ancient geography of single 

 formations, because synthetic maps that embrace several or all the for- 

 mations of a system can not lead to definite results. This is because too' 

 much time is embraced and seas are united that are inconstant, periodic 

 in appearance, and more or less oscillatory in nature. 



THE DIASTROPHIC METHOD 



Under the term diastrophism are included all the movements within 

 the earth's mass and reflected on the surface, remolding the topography 

 of the lands and changing the shape, contour, and size of the oceanic 

 hollows. It is this periodic elevation of the lands and the removal 

 through erosion of the protuberant parts that cause the oceanic strand- 

 lines to shift back and forth over the continents, resulting in continental 

 submergences and emergences, a diastrophic action that underlies a nat- 

 ural classification of the successive geologic events. 



A shifting of the oceanic strand-line is indicated (1) by unexpected 

 changes in the superposed faunas, (2) by the sudden appearance of unre- 

 lated species and genera, (3) by the obvious- breaks in the stratigraphic 



