Permian Formation of Texas. 121 
marks upon the reputed North American Permian, in the course 
of which reference will be made to the bearing which the pres- 
ence of Mesozoic types among the Texan Permian fossils has 
upon the question of the geological age of the strata containing 
them. 
From time to time during the past thirty years there have been 
discussions among geologists as to whether there is in North 
America any true equivalent of the Permian formation of Europe. 
Some writers have been uncompromising in their advocacy of the 
affirmative side of this question, and others have been equally pos- 
itive in asserting the negative. Much of this difference of opinion 
has arisen from imperfect knowledge of essential facts, and much 
from want of a clear definition by the respective writers as to what 
they have regarded as constituting equivalency in this case. Al- 
though much addition has within the past few years been made to 
our knowledge of facts bearing upon this question, and it is evident 
that clearer views upon it are now generally held than formerly 
prevailed, it is too much to expect that the views of all geologists 
should even now fully agree. The following statement of the 
present condition of this question, as the writer understands it, is 
presented that the reader may understand more clearly his views, 
and the reasons for the conclusions and opinions which are ex- 
pressed in this article. 
In Europe the Carboniferous system is understood to be divided 
into three great groups, namely, the Lower Carboniferous, the 
Coal-measures and the Permian, which are definable from one an- 
other, not only by palaeontological, but by stratigraphical character- 
istics. In North America, the great Carboniferous system is quite 
as largely developed as in Europe. The Lower Carboniferous and 
Coal-measure groups are, upon both stratigraphical and palaeonto- 
logical grounds, as clearly recognizable and distinguishable from 
each other, in some parts of this continent, as they are in Eu- 
rope, but the Permian has hitherto had no such undisputed rec- 
ognition. Therefore, the question now to be considered is whether 
the Permian of Europe has really an equivalent anywhere in 
North America; and if so, how that equivalency is recognizable. 
There are seven principal regions in North America within which 
strata occur that have been by different authors referred to the 
Permian. These are (i) southwestern Pennsylvania and northern 
West Virginia ; (2) Prince Edwards Island ; (3) eastern Illinois ; 
