792 J. BAREELL^ MEASUREMENTS OF GEOLOGIC TIME 



The Atlantic coastal plain of the United States may be cited as an 

 illustration of these pj-iiu'iples^ valuable furthermore in showing Iioav 

 nature departs from the simplicity of a diagram. In the Jurassic the 

 transition line between erosion and sedimentation was southeast of the 

 present shore and erosion prevail (id on all t-lie surface now open to obser- 

 vation. ]S'"ear the end of the Jurassic or beginning of the Comanche, a 

 pronounced tilting took place, ac^companied by a northwesterly migration 

 of the axis inland to somewhat beyond the present edge of the coastal 

 plain. As a consequence, the Patuxent formation, of fiuviatile origin, 

 was laid down. Then emergence occurred; that is, the axis migrated 

 eastward, and erosion took place to some extent. Then another rhythm 

 again depressed this region, elevated the baselevel, and the Arundel 

 formation was deposited. Here the deposition was brief, as the forma- 

 tion is found wdthin shallow valleys which had been cut in the Patuxent. 

 Following diastrophic rhythms resulted in the Patapsco, the Earitan, 

 and the Magothy. I'he formations now become marine and the Matawan 

 and Monmoiith follo^^'." Bet^veen each formation is an unconformity 

 which near their landward margins probably represents much more than 

 half of the total time. Besides the diastrophic oscillation superimposed 

 on the tilting, the stratitication of the sediments, as shown in the varie- 

 gated beds, shows many minor rhythms, doubtless in part climatic. 

 During the Cretaceous the shore migrated inland, uplift slackened, and 

 the resulting waste was less. The sedimentation Avas now wholly marine 

 and the record for a time is probably more nearly complete than it was 

 during the Comanche, although the thickness of sediment is less. 

 Possibly the greensands of the Cretaceous represent for a time a con- 

 tinuous record, for these are now found to be developed in quiet waters 

 near the limits of wave action on the outer parts of the continental shelf. 

 The Tertiaiy formations show the continuation of a history of oscilla- 

 tions similar to those shown in the Comanche and Cretaceous, long in- 

 tervals of sea retreat alternating with advances. ' Throughout all this 

 time there seems to have been a progression in tilting, doubtless irregular 

 and spasmodic, but irreversible, combined with complex vertical oscilla- 

 tion of the baselevel in longer and shorter rhythms. 



Let this record on the shore of a continent be compared with the 

 record as made in a geosyncline within the continent as seen in that 

 of the Appalachian trough. The latter throughout the Paleozoic re- 

 ceived sediments from the eastward from the land of Appalachia. The 

 maximum thickness was perhaps 40,000 feet, mostly clastic deposits, 

 but with thick limestone formations intercalated. The sandstones aurl 



