UPPER CARBONIFEROUS 307 
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. 
Although the Productus giganteus fauna strictly speaking is, so 
far as we know, restricted to California, there is another western 
fauna having a different facies which I am inclined to correlate with 
it. It comprises little besides corals, chiefly large Cyathophylloids. 
I have noted it in Utah, Montana, and Idaho. We have some rea- 
son to believe that it represents the Upper Mississippian to the East 
and the Baird fauna to the West. If this is so, the evidence upon 
which we chiefly relied for recognizing post-Mississippian erosion— 
the absence of the Upper Mississippian—is lacking over this area 
and the hypothetical land mass would appear to have extended 
westward on its northern margin no farther than western Montana 
and central Utah. 
As to what were probably the northern and southern boundaries, 
evidence is wanting, Carboniferous strata being absent in Canada 
across its trend and absent or concealed by Cretaceous overlap in 
Mexico except just over the Texas border in the state of Chihuahua. 
The unconformity of which I have just been speaking occurs at 
the base of the Upper Carboniferous. There is, however, a second 
important unconformity which occurs in the middle of the Upper 
Carboniferous and is less widespread as to the area in which it has 
been recognized. Like the other, it is marked rather by overlap than 
by discordance. The overlap is most conspicuous in western Texas 
and New Mexico, but equivalent strata, distinguished from the pre- 
ceding ones by a distinct faunal change, and in some cases by basal 
conglomerates, probably extend into Arizona and Nevada, or even 
farther. 
Lithologically the beds of the Upper Carboniferous and Per- 
mian present the greatest variety, and about the only truth of 
broad applicability has long been known. I mean that in eastern 
North America the sediments of the Upper Carboniferous are chiefly 
shales, sandstones and conglomerates with some thin limestones, while 
in the West the limestones have a much larger development, and 
