476 E. O. ULRICII— CORRELATION OF THE STRAND-LINE 



Yet, despite this fact, pelagic animals as a class are the most valuable 

 of all for intercontinental correlations. Indeed, when it comes to the 

 identification of Lower and Middle Paleozoic horizons across great oceanic 

 basins like the Atlantic and the Pacific, only the graptolites afford rea- 

 sonably accurate results. The ammonoid cephalopods are similarly useful 

 in recognizing later horizons. Both types owe their advantage to com- 

 plexities of structure in which relatively trivial modifications are cer- 

 tainly if not readily determinable. The pteropods also might share in 

 this correlation value were it not for their relatively simple construction. 



GENERIC ALLIANCES AMONG PELAGIC ORGANISMS INSUFFICIENT TO 

 ESTABLISH CONTEMPORANEITY OF DEPOSITS 



However useful in correlation these pelagic animals may be, experience 

 has shown that where exact results are required mere generic alliances 

 are quite insufficient to establish contemporaneity. The genera com- 

 monly prove to have endured too long to be of exact value in detailed 

 correlation. The case of the Utica fauna is an excellent illustration: 



In 1847 Hall identified all dark graptolite-bearing pre-Niagaran shales 

 then known in America as belonging to the Utica age. We will overlook 

 the fact that he included at this time even the Georgia slate, which is 

 now classified as Lower Cambrian; likewise the Levis shale of Canadian 

 age. Neither of these formations was correlated with the Utica in 1862. 

 But on account of the presence of supposedly the same graptolites, the 

 Normanskill and Magog shales in eastern New York and Quebec, the 

 Canajoharie shale in the Mohawk Valley, the Collingwood shale in west- 

 ern Ontario, and the Maquoketa shale in the Mississippi Valley were for 

 a long time after 1862 regarded as of Utica age. In the past 10 years, 

 however, detailed stratigraphic investigations, coupled with a closer study 

 and consequent discrimination of their respective fossils, have shown the 

 Normanskill to be of Upper Chazyan age, the Magog, Canajoharie, and 

 Collingwood of respectively Lower, Middle, and Upper Trenton age — all 

 four, therefore, of greater age than the typical Utica shale — while the 

 Maquoketa was found to be much younger, being an early Silurian — 

 Medinan — formation. 



In the present relatively refined state of knowledge respecting the 

 character and distribution of graptolites the inadequacy of generic al- 

 liances of graptolite faunas in making detailed correlations is fully recog- 

 nized. Besides, it is no longer necessary to depend on such uncertain 

 evidence, because we now recognize many closely drawn species and 

 varieties in widely separated localities; and these minute identifications 

 enable us to correlate graptolite zones with unquestionable certainty. 



