THE RELATIONS OF SOME CARBONIFEROUS FAUNAS 1 7 



to the Trans-Pecos faunas. Neither in California nor in Alaska 

 has the strongly characterized fauna of the Capitan limestone 

 been discovered. The Alaskan faunas and, as already stated, 

 those of the McCloud limestone and the " McCloud shale" can 

 probably be correlated with the Hueconian fauna, with which, 

 indeed, their athnities are chiefly shown. In Alaska, however, 

 there are indications of faunas lower than the Hueconian, though 

 not as yet of any higher. 



The Upper Carboniferous faunas of the west were known to 

 the earlier writers in an extremely sporadic and incomplete 

 manner. In the last 20 years much more extensive and better 

 correlated material has been obtained, which seems on the one 

 hand to indicate a regularity of succession and an extent of dis- 

 tribution at first far from apparent, and on the other to empha- 

 size an unlikeness to the faunas of the Mississippi Valley, which 

 was always more or less obvious. The difference, which is 

 manifested chiefly in the younger Carboniferous faunas, seems 

 too great to be explicable under the hypothesis of merely local 

 conditions acting upon identical faunas in freely communicating 

 seas, and I am disposed to consider that the Western faunas may 

 have had immediate antecedents different from the Eastern, 

 and that they may have been prevented from intermingling 

 with them, either by some terrestrial barrier or by such marked 

 diversity of environment that neither assemblage of species 

 could exist in the habitat of the other. The resemblance to 

 eastern Pennsylvanian faunas seems to be manifested in the 

 West, especially by those having a low position in the secdon, 

 and they are often more or less closely allied with the Pottsville 

 faunas of Arkansas. Therefore it is likely that areal differ- 

 entiation took place after the beginning of Pennsylvanian time. 

 Probably it was at the close of the Pottsville. 



In a recent paper on the Carboniferous faunas of Colorado ' 

 2 facts seemed to develop, namely, that the faunas of that 

 State were of the usual Pennsylvanian type, and that the stratig- 

 raphy and lithology indicate disturbances and shore condi- 

 tions during Pennsylvanian time. This is suggested by the gen- 

 eral sandy and conglomeratic character of the Pennsylvanian 



' U. S. Geol. Surv., Professional Paper 16, 1903. 

 Proc. Wash. Acad. Sci., June, 1905. 



