SUMMARY 



247 



was not checked into the marine sequence. Even now all that we c^n 

 safely state is that it appears more probai)le that the Morrison is of Upper 

 Jurassic age than of early Lower Cretaceous time. 



What caused the withdrawal of the Sundance or Logan sea is not 

 yet known, Init it may have resulted from the elevation of the Sierra 

 Nevada ]\Iountains, a diastrophic movement of great magnitude. In any 

 event, after Sundance time the Pacific Ocean never again spread east of 

 the western Cordilleras. Not only this, but the marginal overlaps of this 

 ocean were smaller in area after Jurassic time than they had been before 

 this period. For a time the writer sought to connect the deposition of 

 the Morrison with the Sierra Nevada crustal movement and thought that 

 it followed the culmination of this diastrophism. Lee, however, empha- 

 sizes the close paleophysiographic similarities of the environments of the 

 Sundance, Morrison, and Purgatoire formations, indicating that the 

 Sierra Nevada movement did not affect the area of these deposits lying 

 far to the eastward. 



From the synopsis presented beyond of the very interesting work of 

 German paleontologists, it is seen that in the Tendaguru of East Africa 

 there are two marine zones replete with guide fossils, and that beneath 

 each one there is a dinosaur horizon having sauropods very similar to 

 those in the Morrison. The whole series is regarded as one of continuous 

 deposition and is replete with interest, not only because of the varied 

 fossils of the sea and land, but further in that we get here a better under- 

 standing than heretofore as to the habitats of the sauropod dinosaurs. 

 The apper marine Trigonia schwarzi zone is clearly of Lower Cretaceous 

 or Neocomian time, while the middle T. smeei zone is of late Jurassic or 

 more definitely of early Upper Kimmeridgian time. Accordingly the 

 German stratigraphers refer the upper dinosaur horizon to the Lower 

 Cretaceous and correlate it with the Wealden of Neocomian age, while 

 the middle dinosaur horizon is regarded as of the older Kimmeridgian. 

 The dinosaurs, and more especially the sauropods, are held, as a result 

 of the preliminary studies, to be very much alike in both horizons, though 

 later on it may be seen that they are somewhat different in each level. In 

 any event, they are very much like those of the Morrison and thus help 

 to fix the time of the American formation. 



As the ammonites of the Trigonia smeei zone are of the older Kim- 

 meridgian and as in the standard sections of western Europe there is 

 still considerable Jurassic time left, it would appear more natural to 

 regard the upper dinosaur zone as filling this time, because of the con- 

 tinuous deposition of the Tendaguru series. This is not the view of the 

 Germans, and the one here presented was first suggested to the writer by 



