464 E. O. ULRICII CORRELATION OP THE STRAND-LINE 



taken of the probably intermediate character of the fauna that must have 

 lived somewhere while the great series of practically unfossiliferous 

 Medinan deposits was being laid down in New York and Pennsylvania — 

 what should be done? Shall we, as advocated by Grabau, Schuchert, and 

 others, revise' the New York standard and break up the practice of three- 

 quarters of a century, a practice that Avas perfectly satisfactory and scien- 

 tific until it ran into a prevailing, though none the less imperfect, faunal 

 conception? Or shall Ave, as I believe Ave should, hold to the New York 

 standard and revise instead our faunal lists, which, as every Avell in- 

 formed student knows, need revision badly enough ? 



DISCORDANT TESTIMONY OF LAND AND MARINE ORGANIC REMAINS 



Paleobotanists and vertebrate paleontologists often differ from the in- 

 vertebrate paleontologist in matters of correlation. Commonly, the latter 

 places the disputed formation higher in the time scale than do the others, 

 but in a feAV cases the opposite has occurred. On investigation, a definite 

 reason for these disagreements is found in the altogether different effects 

 of physical changes on marine and land organisms. 



Land organisms naturally found the conditions that are best suited for 

 their extension and expansion in periods characterized by predominance 

 of sea retreat. Obviously these favoring conditions Avere best deA-eloped 

 during the transition stages between each pair of geologic periods when 

 the land areas attained their greatest expansion and their longest dura- 

 tion. In the very nature of the case the land organisms not only experi- 

 enced the heydays of their existence during the intersystemic emergent 

 stages of the continents, but the particular facies marking each of these 

 must have begun in the sea-retreating closing ages of the expiring period 

 and continued, though with decreasing representations, into the opening 

 ages of the next period, Avhen the ensuing revival of dominant submergent 

 conditions induced another decided change in the composition of the land 

 flora and fauna. 



Apparently this is the reason why the evidence of land plants and ani- 

 mals is commonly not so clearly indicative of the exact age of beds be- 

 longing near — either above or beneath — a systemic boundary as it is in 

 distinguishing upper and loAver formations of the same system. In other 

 words, the greater modifications of the ancient land floras and faunas 

 seem to have occurred in the middle parts of the geologic periods and not, 

 as is the case with marine organisms, between the close of one period and 

 some still early part of the next. 



Disclaiming all intent to discredit the value of land plants and animals 

 in stratigraphic correlation — on the contrary, I regard their specific testi- 

 mony as no less trustworthy than that of the marine invertebrates — it yet 



