304 W. T. LEE MORRISON A CRETACEOUS FORMATION 



be true fo]- marine formatioBS that have been most extensively studied, 

 how much more is it true for non-marine formations whose fossils are 

 often of questionable value in determining age ? 



It is safe to say that periods of erosion, with which might well be 

 grouped periods of non-marine deposition, arc comparable in duration 

 to the recognized periods of marine deposition ; but relatively little atten- 

 tion has been given to them. It would seem tbat a study of ancient land 

 forms, together with a study of the physiographic conditions under which 

 they developed, might lead to the establishment of a time scale that would 

 be valuable for comparison with the scale now used, or at least serve as a 

 check on it. In this connection it is evident that fossil plants are worth 

 all tlie stud}- and consideration they are now receiving, for although they 

 are more difficult to collect than shells, they occur in non-marhie deposits 

 in number comparable to the shells in marine sediments. However, in 

 the absence of an adequate study of paleophysiography it might be well 

 to inquire if it is possible now to find and to apply physical criteria for 

 determining the age of a formation like the Morrison, whose place can 

 not be fixed in a marine conformable succession of deposits and whose 

 age is not definitely indicated by its fossils. I may perhaps be pardoned, 

 therefore, if, with due regard to the faunal evidence which I shall touch 

 on very briefly, I attempt to find criteria outside the realm of paleontology 

 that will aid in (Ictci'inining the age of the formation. 



Faunal Considerations 



It is well known to geologists that Marsh, who described so many of 

 the Morrison dinosaurs, maintained that they prove tlie Jurassic age of 

 the Morrison. The influence of his opinion is still strong, but geologists 

 have gradually l:)een drifting away from it. Marsh and others following 

 him have regarded the IMorrison as essentially equivalent to the Wealden 

 (2, page 591) ; but, although the latter is now generally regarded as 

 Lower Cretaceous, there are many who still hesitate to admit that the 

 Morrison is post-Jurassic in age. The Arundel or middle formation of 

 the Potomac group, as restricted by the Maryland Geological Survey, 

 contains dinosaur bones which, according to the recent studies by Lull 

 (3, page 178), correlate this formation with the Morrison. Berry (4, 

 pages 163-164) also has recently shown that the Potomac plants, which 

 occur mainly in tbe Patuxent formation below the dinosaur beds, are 

 closely allied to the plants that occur in the Kootenai, which is possibly 

 a little younger than ^lorrison (18, page 22).- 



2 There is doubt as to the relation of the Morrison to the Kootenai, but Fisher (18, 

 page 22) has shown that the Kootenai lies with apparent conformity on beds which he 



