i'iUNAL EVIDENCE AND CORRELATION. 255 



vanian. The general ag-e of the base of the Hermosa formation is, fortunately, quite 

 without the range of speculation. The presence of such characteristic forms as 

 Fiisulina cylindnca, Deriya crassa, Chonetes flemiiigi var. verneuilianus, Prodfucttis 

 nel)7'asTeensis, Marginifera wahashensis var., Spirifer cameratus^ Squavi ularia perplexa, 

 and Serwvwiila si(btilita stamps this fauna as Upper Carboniferous. 



The lower Hermosa fauna is probably closely related to that of the Molas forma- 

 tion, though the latter is too imperfectly known to permit much to be said of it with 

 safety. It is also related to the fauna of the middle Hermosa, yet retains a certain 

 individuality of facies which renders its recognition in the San Juan region easier 

 than that of either of the overljdng divisions. The table shows that Ch. flemingi var. 

 ■rerneuiliavus. Product us infiatus. Productiin gcdlatinensis^ and Prodiictus sp. b. do not 

 range into the middle and upper Hermosa, and that Prodatctus semir'eticulatus var. 

 hermomnuK. Productun punctatus^ Spirifer rocky rnontanun^ and a number of other 

 species found above do not occur in the lower. In working through the collection, 

 however, I was more impressed by the abundance, persistence, and distinctive form 

 of Spirifer hoonensls ?, though the latter sometimes occurs abundantly at higher 

 levels, and of the small Productus gallatinensis. Aside from the brachiopods the 

 fauna of the lower Hermosa has little in common with the- division above. Such 

 species belonging to other groups as are found there appear as a rule to be peculiar 

 to it. 



Expressing the relation between the three divisions of the Hermosa formation and 

 the Rico formation numerically, as has been done in other cases, by taking the ratio 

 of the combined fauna of two adjacent divisions to the species common to both, 

 the relation between the lower and middle beds is express^ed by the number 19.2, 

 between the middle and upper by 21.3, and between the uj^per Hermosa and the 

 Rico by 12.8. Similarly, the relation between the Rico formation and the Hermosa'* 

 taken as a whole is 9.9, .somewhat less, as one would expect, than that existing 

 between it and the upper division alone. While this form of comparison is not 

 without significance it is doubtful whether a numerical expi'ession ever adequately 

 represents the relation between two faunas in stratigraphic paleontology, while some- 

 times it is evidently misleading. One defect, which might perhaps be remedied, is 

 that it overlooks the element of abundance in individual local occurrences or per- 

 sistent repetition of local occurrence. The most serious defect is that it expresses in 

 regard to faunas their total similaritj' or difference resulting from the two factors of 

 evolutionar}' development and of the selective action of environmental conditions, 

 while it is for one of these chieflj' that the paleontologist would find expression. The 

 other in most cases could be safely overlooked. At all events, to correctlj^ under- 

 stand the geologic relations of faunas, as well as the geographic, it is necessary 



« Two of the species common to the Rico and Hermosa formations have not been recognized in the upper Hermosa. If 

 they are regarded as having a continuous range and are counted in with the other upper Hermosa species, this relation 

 would be expressed by the inimber 15.4. 



