562 BAILEY WILLIS 



Permo-Carboniferous fauna. This alternation of strata of different lithologic 

 character can be subdivided in great detail when the exigencies of mapping 

 require it. The important lines limiting the Rico formatioii above and below 

 cannot be drawn upon lithologic grounds. If lithologic character alone is 

 relied upon for the subdivision of this great complex embracing strata 

 belonging partly to the Paleozoic and partly to the Mesozoic, the formation 

 units would be of relatively minor importance, and the great historical sub- 

 divisions would not be expressed. The criterion of color applied to this 

 complex groups the Permo-Carboniferous with the Trias. This has been 

 done in previous general discussions of this complex in this region. In fact, 

 the Permo-Carboniferous beds should be grouped with the Carboniferous if 

 the larger elements of time division are to rtceive recognition upon the geo- 

 logic map of the Rico quadrangle. (Italics the present writer's.) 



There are two questions involved : First, what Mr. Cross's 

 map expresses ; second, what it should express. The Hermosa, 

 Rico, and Dolores are clearly not formations in the sense defined 

 in the Tenth A?i?iual Report, for lithologic continuity is divided 

 at the top and bottom of the Rico on the basis of contained 

 fossils. They are intended to be faunal divisions, but their 

 limits are very ill defined, since fossils are commonly so few in 

 the Red Beds that the finding of them higher or lower in the 

 series is a matter of accidental discovery rather than of occur- 

 rence. The map appears, then, to express indefinitely the 

 arbitrary application of a time scale (Paleozoic, Mesozoic or 

 Carboniferous, Permo-Carboniferous, Triassic) to a series which 

 is capable of division into lithologic individuals. Were it so 

 divided it would yield a record of physical conditions, a record 

 which is now obscured by this classification. 



It is an important fact that conditions of erosion and deposi- 

 tion were continuously favorable to the accumulation of red 

 sediments while biologic migration or evolution modified the 

 organisms present from Paleozoic to Mesozoic types. But in 

 discussing the formations their continuity is the essential, just 

 as in describing the faunas their discontinuity would be. To 

 impose the division of the latter upon the lithologic individual 

 is misleading, and to call the faunal units " formations" in a 

 publication for which that term has been accurately defined is a 

 misuse of the word. 



