396 GEOLOGY OF NORTHWESTERN TEXAS. 



any one kind of reason, but shall use all the evidence I have collected. I 

 shall notice the lithological character of the material, the stratigraphic rela- 

 tions, and the paleontological facts bearing upon the question, as I have ob- 

 served them in this district, premising that the relative importance and value 

 of the different kinds of evidence are in my estimation here named in ascend- 

 ing order. 



We need not expect to find as great differences between the Carboniferous 

 and Permian or Triassic and Permian as there is between the Carboniferous 

 and the Triassic, for the former are necessarily more closely related in every 

 respect than the latter, the Permian being the transition period between the 

 two. 



It is very often the case that stratigraphic evidence is the only kind that 

 can be obtained for determining the age of a series of strata. If we find a 

 great difference between the dip of adjoining sedimentary rocks, we nat- 

 urally conclude that some considerable time has elapsed between the time of 

 their deposition. Such is not always the case, however, for great cataclysms 

 occurred sometimes in the midst of a period, causing unconformity between the 

 rocks of the same period. And again, the fact of conformity does not neces- 

 sarily prove that the two formations are of the same age. Lithological evi- 

 dence is much more satisfactory, yet is not as conclusive as one would desire, 

 when unsupported by other evidences. Profs. Fontain and White say upon 

 this very subject:* "Thus, in ascending from a known Carboniferous hori- 

 zon, if we find the coal in abundance in the lower beds and disappearing 

 in the upper, while great masses of limestone and fine-grained red shales come 

 in, surely this would be weighty evidence to show that Carboniferous condi- 

 tions had changed to Permian." 



The evidence from paleontological sources is to me always more satisfactory 

 when the fossil forms are sufficiently abundant than the evidence from any 

 other source. In a transition period like the Permian we will always find a 

 decadence of old forms and an introduction of new ones. I doubt not that 

 if the geological record was complete we should be able to trace the different 

 steps by which the different species have been evolved, and that as these gaps 

 in the geological history are filled up the transition periods will show these 

 intermediate forms between the old and the new. The most that can be ex- 

 pected at the present is to show the commingling of these two forms in the 

 same strata, and their presence thus commingled proves the strata to be 

 neither the Carboniferous proper nor the Triassic, but the transition period 

 between the two, which is the Permian. 



It is a well known fact that certain forms which are abundant in the Tri- 

 assic did not exist in the time of the Carboniferous, and that certain forms 



* Second Geological Survey Pennsylvania, p. 111. 



