NO. 4 PHYSIOGRAPHY OF ROCKY MOUNTAINS — LEE ' 35 



region in late Jurassic time must be taken into account. From all that 

 I have been able to learn the whole region was so near sea-level that 

 an easily recognizable unconformity is not to be expected. 



The fact that little evidence of a hiatus has been found between 

 Sundance and Morrison is not proof either that there is or is not 

 a hiatus. The problem must be solved on such broader consider- 

 ations as change in general physiographic conditions, and their 

 causes ; changes resulting in differences in lithology, and in distribu- 

 tion of the formations; in overlap relations and in the presence of 

 interwedging formations. 



On the principle that a period ends with a maximum retreat of the 

 sea, the Sundance of the Rocky Mountain region should hold a posi- 

 tion in the time scale slightly below the theoretical upper limit of the 

 Jurassic system, and there should be found evidence of a slight hiatus 

 corresponding in time to the wedge of upper La Plata sandstone. 

 This naturally raises a question as to the basis of correlation. 



Comparison with formations previously described and classified 

 calls up the question: What weight should be given in interconti- 

 nental correlation to similar fossil forms and to similar faunas; to 

 identical species and identical faunas ? The question is an old one 

 and will probably never be answered to the satisfaction of all geolo- 

 gists. The uses and perhaps also some of the misuses of such data 

 are familiar. One geologist places great weight on similarity of 

 faunas, and another places little weight on this similarity. Some 

 geologists maintain that the similarity of the Sundance fauna to the 

 Oxfordian fauna is sufficient to fix the position of the Sundance in 

 the time scale at the base of the upper third of the Jurassic system. 

 Others admit that so far as the fossil evidence at present available 

 is concerned, the Sundance might as reasonably be placed near the 

 end as near the middle of the system ; and that the two faunas are 

 separated by such distances that differences in environmental con- 

 ditions and barriers to migration might readily negative close corre- 

 lation on the basis of fossils alone. 



This naturally suggests the query, Can we find criteria for correla- 

 tion that are more reliable than those derived from the fossils ? One 

 method has been proposed which seems attractive, but which in the 

 opinion of many geologists has not yet been adequately tested. It is 

 based on the well-known principle that a movement of any con- 

 siderable part of the mass of the earth is likely to change the capacity 

 of the ocean basins and therefore to shift the strand line. A dis- 

 turbance in the solid mass of the globe in one place, such as the 



