PRINCIPLES OF STRATIGRAPHIC CORRELATIONS 513 



The same is true also respecting dissimilar faunas in lithologically 

 similar strata whose contemporaneity may have been suggested by ap- 

 parent identity of stratigraphic position. I think this is so of the upper 

 Chazy faunas contained in the Valcour limestone in the Champlain 

 Valley and the corresponding Holston limestone in east Tennessee, on the 

 one hand, and the very different faunas found in the upper Stones Eiver 

 in the western part of the Appalachian Valley and in the equivalent 

 Pamelia limestone in central Few York. These four formations usually 

 seem to occupy the same stratigraphic interval, being in each case limited 

 below by rocks of middle Chazy age and above by calcareous beds of Black 

 River age. But we know that the Pamelia and the upper Stones River are 

 not of the same age as the upper Chazy formations, since it has been 

 proved by actual superposition at Mercersburg, Pennsylvania, and in 

 Hawkins County, Tennessee, that limestones agreeing closely with the 

 typical upper Chazy are younger than the upper Stones River and older 

 than the Lowville which follows in the same section. 



In practice many paleontologists subscribe, latterly perhaps unwit- 

 tingly or because it had become a habit, to the indefensible belief that the 

 extinction of a species, genus or fauna in a given continental basin usually 

 means its extinction everywhere. In fact, however, this can be true only of 

 species or faunas evolved in and confined to the basin. Moreover, as argued 

 on pages 495-501, such local evolution and development seems to be the 

 exception and not the rule. If the latter supposition is well founded and 

 if the evolutionary stages of faunas as known to us in the fossil state 

 were usually accomplished in oceanic basins permanently inhabited by 

 them, then it is clear that their extinction in any continental basin may 

 be merely as episode in the history of the species or of the life association 

 of which it is a part. At the same time it follows that the partially extin- 

 guished species or faunas continued their existence in their permanent 

 oceanic habitats until perchance another opportunity to invade the same 

 or some other continental basin was offered. Granting the foregoing it 

 is reasonably conceivable that under relatively favorable circumstances 

 the process may have been repeated over and over again. Indeed, the 

 . already many known recurrences prove that something of the kind took 

 place frequently. Such considerations lead to the following principles, 

 8 to 11, which are intended chiefly to account for difficulties in correla- 

 tion arising from fauna! recurrences. 



(8) Recurrent species and faunas. — Tlie possihility of recurrence of 

 species and faunas must ever he guarded against. When properly in- 

 terpreted, the successive appearances may be used to great advantage in 



