METHODS OF PALEOGEOGRAPHY 437 



tinental seas are not only enlarged, but seas are even brought into existence 

 where none whatever occurred. 



Methods of Paleogeography 



classification of methods 



The methods or principles for determining the relation of ancient seas 

 and lands are four in number: (1) The Paleontologic, (2) the Areal- 

 Geologic, (3) the Petrologic, and (4) the Structural or Diastrophic. 



PALEONTOLOGIC METHOD 



Basis of chronology. — The primary basis for a geologic chronology is 

 furnished by the organic remains entombed in the stratified rocks. All 

 methods for the exact determination of geologic time are at present de- 

 pendent upon paleontology, yet it is not to be denied that locally other 

 means than the paleontologic may become of prime importance. 



Continental deposits. — There is as yet no established petrologic method 

 whereby marine deposits can be distinguished from those of fluviatile or 

 lacustrine waters. Fossils, therefore, are of vital importance in determin- 

 ing whether a sedimentary deposit ( 1 ) is derived from waters of the land 

 deposited on the land — that is, a continental deposit — or (2) is of terres- 

 trial origin, but laid down in marine waters. The physical nature alone 

 of some formations renders their fresh-water or eolian origin fairly cer- 

 tain. When with these characters are combined the lessons to be learned 

 from the entombed fossils, then the evidence is convincing. 



The Triassic sandstone of the Connecticut valley consists of deposits 

 which have been termed estuarine and fluviatile. They are better exposed 

 and more extensively quarried in Connecticut than any other formation, 

 and have been explored by geologists and paleontologists for more than a 

 century, but not a trace of a marine animal has thus far been discovered. 

 Evidence equally as strong was available twenty years ago, yet Dana 

 taught that these sandstones were laid down in an estuary like that of the 

 bay of Fundy. If this were true, marine fossils would have often been 

 brought to light. On the other hand, indications of land plants and land 

 animals are frequently met with, the most abundant being the footprints, 

 or autogrammes, as Newberry called them, of quadrupedal, but mainly of 

 bipedal, terrestrial reptiles. After being persistently collected, these 

 tracks were described, first by E. Hitchcock, later by C. H. Hitchcock, 

 while recently this subject has been restudied by Professor E. S. Lull, 32 

 who has determined 40 genera and 92 species. These impressions are all 



32 Lull : Memoir of the Boston Society of Natural History, vol. 5, 1904, pp. 461-557. 



