METHODS OF PALEOGEOGRAPHY 439 



ing periods of calcareous deposition there is much evidence of shrinkage 

 cracks on extensive mud flats that have been subjected to periodic inunda- 

 tions of calcareous material nearly devoid of life. These Paleozoic shores 

 of Appalachia were not unlike the coral tidal flats of present Antillia. 

 In the New York basin, most of the later Paleozoic deposits are those of 

 estuaries, for the material is mainly sands and muds practically lacking 

 in marine life. Here and there occur land plants, Old Eed fishes, and 

 occasionally fresh-water bivalves. Often the sands are red, oxidized, the 

 estuaries of extensive rivers dried out by the sun and air. In the Ohio 

 basin, but more particularly in the Indiana basin, where the adjacent 

 lands were very low, more often occur similar widespread beds indicative 

 of shallow seas. Such are the thin bedded limestones in alternation with 

 shales, dolomites, oolites, and calcareous shales. Here may also be seen 

 ripple-marks and shrinkage or sun-cracks. Along Appalachia and Lau- 

 rentia are occasionally found salt and gypsum horizons, while in the late 

 Pennsylvanic seas of Oklahoma occur alternations of red shales with thick 

 beds of gypsum. The intraformational conglomerates further point to 

 shallow agitated seas. 



The superficial depth of the continental seas is also reflected in the end- 

 less formational names, based on petrologic change, which have been pro- 

 posed by the areal geologists. For further discussion of this subject, see 

 the paper by Ulrich elsewhere in this volume. 



Fossils indicative of exact time. — Most paleontologists are now aware 

 that fossils can be relied upon to determine not only the broader units of 

 time, but horizons of considerable areal extent, with but a few feet of 

 thickness as well. Elsewhere in this volume Ulrich introduces remarkable 

 examples of thin horizons that may be identified by a few fossils scattered 

 over vast regions. This clearly indicates the increase in paleontologic 

 knowledge since the days of Strata Smith's teachings, and today in Amer- 

 ica stratigraphic results are more easily attained because of the wide ex- 

 tent of successive undisturbed formations. Geological faunas within given 

 provinces practically appear instantaneously, and intercontinental correla- 

 tions based on faunule identities: — that is, a combination of several spe- 

 cies, in most cases, will probably not be out of synchrony more than from 

 10 to 20 feet. The evidence for these statements is discussed elsewhere 

 by Ulrich. 



It is now not always necessary to possess large collections of fine mate- 

 rial in order to make correlations or to identify horizons of limited zones, 

 a single fossil often being sufficient for this purpose. Brachiopoda and 

 Bryozoa are usually to be found in the Paleozoic and are the finest hori- 

 zon markers. In the Cincinnati series, Catazyga headi determines a zone 



