348 T. W. STANTON INVERTEBRATE FAUNA OF THE MORRISON 



was a comparatively brief episode in the long period covering a large part 

 of the Mesozoic era dnring which continental conditions prevailed. If 

 the retreat of this sea was immediately followed hy a long period of uplift 

 and erosion prior to Morrison time, the evidence of it seems to have been 

 overlooked by geologists. There is evidence that there was erosion and 

 baseleveling before the marine Jurassic was laid down. On the assump- 

 tion that marine waters were finally withdrawn from the Sundance sea 

 either by a slight general uplift or by the silting up of the shallow basin, 

 it is not unreasonable to suppose that the formation of continental de- 

 posits like the Morrison might begin almost immediately over the same 

 area and extend far beyond it. The alteniative hypothesis requires a 

 considerable interval between the Sundance and the ^rorrison ap])arently 

 unrepresented by either deposits or erosion. 



WASHITA FAUNA 



It is not within my province to speak of the possible upper limit of the 

 Morrison as determined by the age of the floras in the overlying Kootenai 

 and Lakota formations of Montana and the Black Hills. The oldest 

 overlying marine fauna is found above the southern extension of the 

 Morrison in southeastern Colorado, northeastern New Mexico, and north- 

 western Oklahoma, where the Morrison, with its characteristic dinosaurs, 

 is overlain by beds containing the fauna of the Washita group, which is 

 the uppermost of the three groups forming the Comanche series. In 

 America it has been customary to classify the Comanche series as Lower 

 Cretaceous in contrast with the great Upper Cretaceous series beginning 

 with the Dakota sandstone. A number of European paleontologists who 

 have given some attention to the Washita fauna l)elieve it to be of Ceuo- 

 manian age, the Cenomanian being regaidrd as tlie base of the Upper 

 Cretaceous in Europe. This determination being taken as the latest 

 possible assignment of the overlying beds, the Morrison must be older 

 than Cenomanian and probably younger than Oxfordian. That it repre- 

 sents all of tliis interval is not probable, but in my opinion the lithologic 

 and stratigraphic evidence of a break in sedimentation is fully as great, 

 if not greater, between the Morrison and the rocks of Washita age, where 

 they are in contact as it is between the Moi-rison and the Sundance in the 

 northern area where these two formations come together. So far as 

 stratigraphy and invertebrate faunas are concerned, the Morrison is some- 

 what more likely to belong to the diirassic ])()rti()n of the interval just 

 indicated than to the Cretaceous portion; but tbcir e\ idence is not con- 

 clusive on this point. 



