METHODS UNDERLYING A DETERMINED CHRONOLOGY 499 



foreign pebbles are of greatest aid, and next after them the lateral tran- 

 sitions from marine into brackish and fresh water deposits. Cleanly 

 washed quartz sandstones are usually good indicators of shallow seas and 

 of nearness of land, but not necessarily of actual shore conditions. On 

 the other hand, it is probably true that in the great majority of marine 

 formations the shore and shallowest water deposits have been eroded back 

 after each emergence, and for distances commonly up to 30 miles. In 

 the Devonian of the Appalachian delta the shore deposits are now absent 

 over an area of from 50 to 75 miles in width, and in the Cretaceous of 

 New York and Massachusetts they are gone in a strip of land that is 

 between 75 and 90 miles wide (Barrell). 



THE PALEONTOLOGIO METHOD 



The primary basis in correlating strata and arranging them in a chron- 

 ologic sequence is the fossils that were entombed at the time the rocks 

 were formed. Their succession, and therefore their stratigraphic value, 

 have been determined from the superposition of the strata. With the aid 

 of fossils we are also enabled to trace a formation from place to place, 

 and thus to work out considerable of the geography of the time as well, 

 and, finally, from a succession of formations, the times of diastrophic 

 movements. On this occasion it is not my privilege to dwell on the 

 paleontologic method, and I will take this opportunity to present only a 

 few points that bear more especially on the making of paleogeographic 

 maps. 



A little insight into living faunas shows that their combination varies 

 from place to place, and that while a few species are limited to a very 

 restricted geographic range, the greater number have a more or less wide 

 distribution. Among the marine invertebrates many of the sessile and 

 semi-sessile forms are localized in a definite sedimentary facies; others 

 are not so circumscribed, while the free species have a tendency to spread 

 extensively. The degree of radiation is, however, very variable with each 

 species, but in general the tendency is toward wide distribution. 



Eegarding the dispersal of living marine mollusks, Ball has presented 

 very valuable data that have a direct bearing on the fossil faunas. He 

 states that the Peruvian warm-water province extending from Guayaquil, 

 Ecuador, to southern Chile has over 800 species of bottom-dwelling mol- 

 lusks. In a straight line this province has a shoreline that is over 2,000 

 miles long, and yet more than 55 per cent of the species have a greater 

 range, extending into the adjacent areas, while 40 per cent are restricted 

 to the region. We may therefore say that about 60 per cent of living 

 marine bottom-dwelling invertebrates have a range of between 2,000 and 



