STRATIGRAPHIC EVIDENCE 331 



The vertebrate paleontologists apparently would place the top of the 

 Lance at the highest point at which dinosaurs have been found; but 

 unfortunately in those areas — and there are very many of them— where 

 no dinosaurs are known the Lance will be without a known top, and even 

 where they do occur this sliding scale makes the fixing of the boundary 

 dependent on the accident of discovery, and the point accepted as the 

 line today may be very different from that necessitated by the work of 

 tomorrow. 



The invertebrate paleontologist would fix the top of the Lance by the 

 highest point at which marine invertebrates ha\^ been found. This cri- 

 terion is even more limited in its application than the last, since marine 

 invertebrates are known from a comparatively small area in North and 

 South Dakota. 



It is obvious, therefore, that the fixing of this boundary has a practical 

 aspect that must be considered, as well, as the technical, stratigraphic, 

 and paleontologic sides. If we subordinate the taxonomic significance of 

 this pre-Lance datum plane as certain paleontologists would do, we sub- 

 stitute for it a criterion of vague, obscure, and unequal application. 

 "Xext to natural relationship the quality of convenience is the prime 

 desideratum in stratigraphic taxonomy," says Ulrich in discussing the 

 Ordovician-Silurian boundary; and he continues, "Let us, then, be rea- 

 sonable and practical and accept with proper valuation these diastrophic 

 boundaries, which nature has most clearly and widely indicated." 



Paleobotanical Evidence 



I am possibly prejudiced when I say that to my mind the paleobotanical 

 evidence is convincing and of the highest importance. 



First, as to the evidence it affords regarding the distinctness of this 

 time break : In the Eaton Mesa region the Vermejo formation, the first 

 beneath the unconformity, has a flora of 108 species. The Eaton forma- 

 tion, the. one next above the break, has a flora of 148 species. Only four 

 species have been found in common. In the Denver basin the Laramie 

 has a published flora of 07 si)ecies. six of which are found also in the 

 Eaton flora and 10 in the Arapahoe and Denver floras. In Carbon 

 County, Wyoming, the '"Lower Laramie" (true Laramie, in the wdter^^s 

 opinion) has a flora of "about 50 species, five or six of which , occUr aUo in 

 the dinosaur-bearing beds above, which has a flora of about 70 species. 

 Throughout the vast area over which the Lance formation is known there 

 have been reported 16 species of plants that come into its flora from beds 

 below the unconformity — that is, from the Laramie, Montana, etcetera. 



On eliminating the duplications in these several lists, as well as the 



