578 MESOZOIC FLORAS OF UNITED STATES. 



As an instance of the superposition of beds of nonmarine tj^pe con- 

 taining the Potomac flora on marine Jurassic beds the Hay Creek strata 

 of Wyoming may be mentioned. In the Hay Creek region of Wyoming 

 there are fresh-water and marsh deposits that contain a well-marked 

 Lower Potomac flora. These rest unconformably on Jurassic marine 

 strata. In the Black Hills, also, we find nonmarine deposits containing 

 the Lower Potomac flora resting on marine Jurassic beds. 



We have direct evidence of the Neocomian age of the Lower Potomac 

 flora in the following cases: 



The Glen Rose beds of the Trinity group in Texas have yielded a 

 number of fossil plants belonging to the Lower Potomac flora. These 

 fossils occur in a lens of fine sediment, in a chalky calcareous mass that 

 abounds in marine Neocomian invertebrates. Professor Marsh, from the 

 comparison of the Atlantosaurus or Como beds of Colorado and Wyoming 

 and their contained vertebrate fauna with the Lower Potomac of Mary- 

 land, was led to think that the}^ are both of Wealden age. It is inter- 

 esting to find that Mr. Willis T. Lee has traced these deposits, bearing 

 the name "Morrison formation," southward until they nearly make a 

 junction with the Trinity beds of Texas, described by R. T. Hill. He 

 makes this statement :° 



According to Mr. Hill's section the Lower Cretaceous, consisting of the Trinity, 

 Fredericksburg, and Washita, lies between the Red Beds and the Dakota. If Mr. 

 Hill's section represents correctly the age of the formations in the Canadian Valley, 

 then the shales and possiblj^ the Exeter sandstone must be of Lower Cretaceous age. 

 But the shales, as I have alreadj^ shown, are probably the same as the dinosaur- 

 bearing shales of the Purgatory. There is some probability therefore that the 

 Morrison formation may be identical \vT.th some part of the Lower Cretaceous of 

 the Texas region. 



In the Shasta group of California the Lower Potomac is well repre- 

 sented and here a Neocomian invertebrate fauna accompanies it. 



Dr. J. Felix found in central Mexico, in the Cerro de la Virgen, 

 near Tlaxiaco, fossil plants of the Lower Potomac in a formation con- 

 taining numerous animal fossils, which, in the opinion of Doctor Felix, 

 fully prove the Neocomian age of the beds. Doctor Felix sent a small 

 collection of the plants to Doctor Nathorst for determination.* The 



a Joum. Geol., Vol. X, No. 1, 1902, p. 57. 



6 Nathorst, in Felix & Lenk, Uebersicht iiber die geologischen Verhiiltnisse des mexicanischen Staates 

 Oaxaco, Beitriige zur Geologie und Palilontologie der Republik Mexico, von .J. Felix und H. Lenk, II. TheU, 

 I. Heft, Leipzig, 1893, pp. 51-54. 



