METHODS UNDERLYING A DETERMINED CHRONOLOGY 501 



this record will lead uui only 'to a determined paleogeography, but also 

 to a determined chronogenesis and phylogenesis. 



The first guidance in the making of paleogeographic maps is found in 

 the widely accepted postulate that the great ocean basins and the conti- 

 nents are to all intents and purposes permanent features of the earth's 

 surface. However, even though the great continents have always been 

 where they now are, their size and shape have not always been constant; 

 some have been enlarged and others have been decreased and added in 

 part to the oceanic areas. 



Another essential in the making of these maps is the determining of 

 the ancient coastlines and the sources of the sediments. Along the Pacific 

 border it is natural to assume that we must be dealing in the main with 

 epicontinental or shelf seas — in other words, with marginal overlaps of 

 the Pacific Ocean — and that therefore the shorelines must lie to the east 

 of the overlapping oceans. It is undoubtedly true that the most easterly 

 shoreline of Pacific waters washed Cascadia, 2 but it does not follow that 

 it was the strand of the open ocean; nor does it follow that Cascadia must 

 have stood on the edge of the continent, for a study of the position of 

 geosynclines leads to the opinion that this ancient land in Mesozoic time 

 faced a narrow, but long and subsiding, trough that was bounded on the 

 west by a marginal strip of probably long islands. The postulated paleo- 

 geographic condition along the west coast of North America during the 

 Mesozoic was probably similar to the geography of the present peninsula 

 of Lower California and the Gulf of California. Better examples are 

 seen off the coasts of eastern Asia. Here the Sea of Japan, a greatly 

 overdeepened geosyncline, is bounded on the east by the Sakhalin and 

 Japanese drowned mountain chains, which together have a length of more 

 than 2,000 miles, while the China Sea is bounded by Formosa and the 

 Philippines, with a length of nearly 1,500 miles. On the other hand, the 

 coarse character of the Mesozoic deposits, and especially the great thick- 

 nesses of the formations, combined with much volcanic material that is 

 spread all along the Pacific border of North America, are in harmony 

 with the theory that these sediments were laid down in a geosyncline, 

 often having high lands bounding the trough on either side. We are 

 further guided to the shorelines by the increasing coarseness and thicken- 

 ing of the formations, and more particularly by the transition of faunas 

 from marine to brackish and fresh water types. 



Having ascertained the significant guide fossils of a fauna and their 

 stratigraphic range in the sequence, the next point of value is the de- 

 ciphering of the geographic distribution of the formation containing the 



2 Bull. Geol. Soc. Am., vol. 20, 1910, pi. 49. 



