UPPER CARBONIFEROUS 125 



Mississippian age and whose upper has furnished, according to Meek, 

 a long hst of Pennsylvanian species. The presence of this uncon- 

 formity is to be detected therefore not always by local evidences of 

 erosion or changes in sedimentation, but sometimes only by paleon- 

 tologic evidence in the abrupt and great change in the faunas and 

 floras, and by stratigraphic evidence in the overlap, sometimes appre- 

 ciable only by considering rather wide areas. 



Inasmuch as we find this extensive area in which a hiatus exists 

 at the base of the Pennsylvanian, and inasmuch as over part of the 

 area unmistakable evidence of erosion is found, the inference is prob- 

 ably a safe one that everywhere within this region the hiatus is partially 

 at least due to post-Mississippian erosion. 



The presence of this erosion period implies the existence of a land 

 surface over the eroded area, for the alternative hypothesis of sub- 

 marine erosion may probably be disregarded. 



The boundaries of the land cannot be exactly defined. On the 

 east I would judge that it must have followed a presumably irregular 

 line southwestward from northern Pennsylvania to southwestern 

 Texas. At least, there is a well marked unconformity west of such a 

 line, while in some sections east of it, sedimentation appears to have 

 been continuous from the Mississippian into the Pennsylvanian. An 

 estimate of where the boundary lay on the western side is conditioned 

 somewhat by our correlations of the western Mississippian faunas 

 with the eastern and with one another, especially as to areas over 

 which the Upper Mississippian is wanting. 



It is pretty well established by many observations that faunas with 

 a Kaskaskian facies are not known west of the Mississippi Valley.' 

 There are, however, some faunas peculiar to the West which may 

 be of Kaskaskian age. The best known and most notable of these 

 occurs in the Baird shale of California, and has not been found else- 

 where on the continent. It is characterized among other things by 

 the European Productus giganteus, and can be correlated more easily 

 with the Mountain limestone of Europe than with our own Mississip- 

 pian. 



' Since this statement was written there has come to hand from the Wasatch 

 range in northern Utah, a fauna having much in common with the upper Mississip- 

 pian of the East. Among other species it contains Productus Cestrieusis in abundance. 



