124 Permian Formation of Texas. 
tion of types would be found on this continent that occur in Europe 
and elsewhere. 
Much difference of opinion has prevailed even among those who 
recognize the importance of considering all the facts which bear 
upon a given case of assumed equivalency. Some have believed 
that it should be strictly chronological as regards the whole of a giv- 
en formation ; while others claim that the most we can reasonably as- 
sume in any case is approximate contemporaneity, and all that we 
can ever certainly know in such cases is the homotaxial relations 
of formations in different parts of the world respectively. The 
scope of this article, however, will admit of only a partial discus- 
If all the time which is represented by the entire Carboniferous 
system in Europe is represented by the entire Carboniferous system 
of North America, the Permian of Europe must necessarily have a 
complete time equivalent somewhere on this continent If that 
system is everywhere incomplete at the upper limit on this conti- 
nent, and the same is complete in Europe, it necessarily follows 
that the stratigraphical time equivalent of the European Permian 
is either absent or incomplete in North America. But all the 
known facts which bear upon this case are of such doubtful value 
in their application to the question of strict chronological equiva- 
lency that it seems to be unprofitable to discuss it. Therefore the 
only question that remains to be considered in this connection is 
that of homotaxy. 
The question, even after being reduced to these limits, is a com- 
plex one, for it still involves the consideration of conflicting and 
disagreeing palseontological evidence as well as a recognition of 
upper and lower delimiting boundaries of the formation. There 
can be no good reason for doubting that there are in various parts 
of North America strata which are homotaxially equivalent, at 
least in part, with the Permian of Europe. But it is equally true 
that much of the reputed North American Permian cannot be satis- 
factorily separated from the Coal-measures, and even those which 
have been separated more or less satisfactorily, are found to be so 
intimately related to the Coal-measures as to make the lower litn'^ 
indefinable." 
writers have applied the compromising term " Permo-Carboniferous" to that 
definable upper portion of the Carboniferous system. Unfortunately, howev ^. 
some American authors have of late applied the same term to the whole Carbon 
