300 GEORGE H. GIRTY 
Lower Mississippian limestones. On the other hand, evidences of 
erosion are often wanting and sometimes any physical suggestion of 
an interruption in sedimentation. A striking instance of this sort 
occurs in southern Arizona. In the Bisbee area limestones of Pennsyl- 
vanian age rest upon limestones of Lower Mississippian age, the two 
series being extremely similar in physical characters, though carrying 
different faunas. The same condition probably exists in the Redwall 
limestone of the Grand Canyon region, whose lower part is of lower 
Mississippian age and whose upper has furnished, according to Meek, 
a long list 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 
