246 c. schuchert morrison and tendaguru formations 



Summary 



This study was begun with the idea that the conclusion woukl be the 

 certain transferal of the Morrison to the Lower Cretaceous or (^omanchian. 

 The writer^ from a preliminary study made two years ago of some of the 

 results of the findings in East Africa, was led to tliis belief, but as the 

 final study progressed it became plain that if the Morrison could be shown 

 to be of Comanchian time at all it would have to be referred to the earliest 

 part of this period. To make certain of the facts, the writer took up 

 correspondence with several of his friends, resulting in the conclusion 

 that for the present the Morrison is best left in the Jurassic, where it 

 has been placed by most American paleontologists. 



The marine Sundance beneath the Morrison has several forms of am- 

 monites, one of which is Cardioceras cordiforme, a species of great strati- 

 graphic significance. If it is a genuine Cardioceras, its age is early Upper 

 Jurassic (Corallian = Argovian), and if a Quenstedtocetas. then latest 

 Middle Jurassic (Oxfordian = Divesian). On the other hand, it appears 

 to be definitely excluded from Amoeboceras, and hence from the Kim- 

 meridgian series. Accordingly, the Sundance is either of latest Middle 

 Jurassic or earliest Upper Jurassic. There is, therefore, in the Great 

 Plains country a long time interval before marine strata are again met 

 with in the Purgatoire formation, one of the members of the A¥ashita 

 series of the Comanchian. In this interval was deposited the Morrison 

 formation of fresh-water origin. 



Until a few years ago it was widely held that the Morrison lies with 

 unbroken sequence on the Sundance, and since the latter is of either late 

 Middle or earliest Upper Jurassic time, it followed, since they were in 

 continuous deposition, that the former must also belong to this period. 

 Now most stratigraphers are agreed that the two formations are not bound 

 together by transition beds, and, further, that the Morrison is a trans- 

 gressive series of fresh-water strata over the marine Sundance. In many 

 places the two formations do not follow one another and the Morrison 

 has a strikingly different geographic distribution from the Sundance. 

 The latter extends, ever widening in area, to the north and northwest 

 into the Pacific, while. the Morrison distribution is greatest in the oppo- 

 site direction, widening to the south. Pardee has recently shown that 

 there is no Morrison in the Garrison and Phillipsburg quadrangles of 

 Montana, and that the Kootenai here rests directly on the Sundance. 

 Therefore in correlating the Morrison we have no means of determining 

 the length of time represented by the break or disconformity between it 

 and the Sundance, and until recently the organic content of the former 



