126 GEORGE H. GIRTY 



Although the Productus giganieus 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 

 coals, which toward the East play so important a part, if not in thick- 

 ness at least economically and significantly in the Carboniferous 

 sediments, are there practically absent. From this it has been justly 

 inferred that the character of the eastern Carboniferous indicates 

 shore and estuarine conditions of deposition, while that of the western 



