586 C. SCHTJCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



In the Lower Horsetown the following are the more distinctive forms : 

 Trigonia cequicostata (also in Upper Horsetown). Phylloceras onoense, 

 Hoplites remondi, Olcostephanus traski, Desmoceras lioffmani (also Upper 

 Horsetown), Crioceras percostatus. and Ancyloceras remondi. In the 

 TJpj^er Horsetown, among other forms occur Ancyloceras lineatus, Haplo- 

 ceras breweri, Lytoceras sacya, and Schlceribachia inflata. The Horsetown 

 is correlated with the Aptian and Gault. Locally it seems to be deficient 

 in continuous deposits into the higher Chico (L'pper Cretacic), but Stan- 

 ton regards this time break as of short duration. At least 10 of the Upper 

 Horsetown species pass into the Chico of Cretacic time. 245 



Continental deposits. — In eastern North America there are no marine 

 Comanchic strata. The Potomac series is fluviatile or fresh water in 

 character. The lower and thinner division, or the Patuxent- Arundel, is 

 often correlated with the Morrison, which is generally referred to the late 

 Jurassic or Wealden (see the Jurassic chapter). Berry has informed 

 the writer that the Arundel is only a different phase of the Patuxent. The 

 former lies unconformably upon the latter, but no marked value should be 

 placed on this physical feature because of the fluviatile character of the 

 formations. Furthermore, the iron-ore bearing Arundel is almost re- 

 stricted to the region between Baltimore and Washington, and the plants 

 are those of the Patuxent. Besides large cycads, the Arundel has yielded 

 dinosaur bones pertaining to PI euro coel 'us nanus, P. alius, Priconodon 

 crassus, Allosaurus medius, and Coelurus medius. The dinosaur remains 

 are in harmony with those of the Morrison and TTealden, and according to 

 present evidence these deposits are as well, or even better, placed in the 

 Comanchic (also see page 582). 



The higher division of the Potomac series is known as the Patapsco, 

 having a rich flora in which the angiosperms originate. This flora, how- 

 ever, is still mainly ancient in aspect — that is, it consists chiefly of ferns, 

 cycads, and conifers. The angiosperms do not become dominant until the 

 Raritan and Dakota of the Cretacic. 



In the northern Great Plains area are other fluviatile beds, first, de- 

 scribed by G. M. Dawson as the Kootenai formation of Alberta. The 

 thickness is given as about 4,700 feet, yet in Montana it is less than 600 

 feet. Locally it is coal bearing, there being in Alberta 22 workable coal 

 beds. The small flora may be compared with the Lower Potomac. There 

 are also a few Unios and fresh-water gastropods, "mostly of simple mod- 

 ern types." 



1245 Diller and Stanton : Bull, of the Geological Society of America, vol. 5. 1894, pp. 

 435-464. 



