56 BULLETIN 169, UNITED STATES NATIONAL MUSEUM 



The kno^vll faunas of this field thus seem to cover most of or all 

 the Middle Paleocene and to continue, probably without a marked 

 break, into the early Upper Paleocene. There are here at least 

 2,000 feet of sediments of about the same type above the highest 

 level with identified mammals, and sediments of somewhat different 

 character in contiguous areas might raise this figure to as much as 

 4,000 feet. These beds doubtless represent most of or all the con- 

 siderable span of later Upper Paleocene time not represented by 

 faunas here, and they may well run into the true Eocene. 



The tentative arrangement based on these correlations is summed 

 up in the diagram on page 55. 



Such a chart is necessarily excessively simplified, so that it does not 

 adequately represent abundant and exact evidence where this is 

 available; at the same time it must be explicit, so that in places it does 

 not reveal the extent of uncertainty and lack of evidence. It is a 

 correlation of strata by their known faunas, never entirely coextensive 

 with the strata themselves. The "faunal breaks" listed definitely 

 imply intervals of time not represented by known faunas, but it does 

 not follow that strata of these ages are necessarily lacking in the region 

 in question. The same is true of the parts outside the known distri- 

 bution of faunas that are labeled "no mammals known." 



ECOLOGY 



NOTES ON THE NONMAMMALIAN BIOTA 



Despite the abundance of nonmammalian fossils in the field and 

 the existence of good collections made by Silberling and others, this 

 part of the biota has not been adequately studied for this area. It 

 is beyond the scope of the present work to include any critical exami- 

 nation of the material or to attempt complete floral and faunal lists, 

 nor has it seemed necessary to delay publication until such special 

 studies could be made. In the present section a few previous records 

 are mentioned, and beyond this the only purpose is to suggest the 

 broad outlines or general character of the biotic background of the 

 mammalian faunas. 



Plants. — A number of small collections of leaves have been made in 

 this field, but as far as I laiow no attempt to collect and to classify 

 its whole flora has been made. Ejiowlton (1902) has published a 

 special note on one small collection, and several other species are listed 

 incidentally in the literature. 



Knowlton (1909, p. 194) listed a few plants from the "lower mem- 

 ber of the Fort Union" (by which he usually means the Hell Creek or 

 La.nce), 1,000 feet below the top. As it is not clear what he considered 

 "the top", this is not explicit. He states that they were "about 1,200 

 feet below the small mammals now being studied by Mr. Gidley", 

 which probably means the Silberling Quarry and would (by my esti- 



