PRINCIPLES OP STRATIGKAPHIC CORRELATIONS 509 



occur at all it is usually in sufficient abundance to insure high rank in 

 the matter of dominance. 



The numerically variable distribution of two varieties of Platystrophia 

 biforata or lynx and of Orthorhyncula Unneyi may sufficiently illustrate 

 the second case. Thus, there are two well defined zones in the hills back 

 of Cincinnati, Ohio, and Madison, Indiana, in which large varieties of 

 Platystrophia are found. The upper one (Mount Aul)urn bed), at the 

 top of the McMillan formation, contains the short-hinged variety, while 

 the lower zone, in the upper part of the Fairview limestone, is marked 

 by the long-hinged, quadrangular variety. Now, at Cincinnati the 

 upper variety is exceedingly abundant, while the lower form is compara- 

 tively rare. At Madison, on the contrary, the lower variety is found by 

 the thousand, while the short-hinged, later variety is far less abundant. 

 Eegarding Orthorhyncula Unneyi, this is a common and highly charac- 

 teristic fossil of the upper part of the Fairview limestone in central 

 Kentucky south of Kentucky river. At Cincinnati, however, this fossil 

 is exceedingly rare, though many of its principal associates in central 

 Kentucky are common enough. 



The Silurian and Devonian coral faunas offer some very notable in- 

 stances of irregular areal distribution. This is true especially of the 

 reef builders, whose colonies, being confined to waters of certain depths 

 and very slightly varying temperatures, vary rapidly in thickness and 

 may pinch out in short distances. Broader differences in distribution 

 of corals and other organic types, as for instance in the Onondaga fauna, 

 as developed in southwestern Illinois on the one side and in central Ken- 

 tucky and New York on the other, probably are due to another circum- 

 stance, namely, the latter areas are near the southeast and northeast 

 shores of the Onondaga sea, while western Illinois is near its south- 

 western shore. The corals invading the basin from the middle Atlantic 

 by way of the Mississippi embayment probably followed the most natural 

 route, which would be the east shore. The relative paucity of the coral 

 element in the fauna of the Grand Tower limestone is thus explained. 



(4) True guide fossils. — A single species, or preferably tiro or three 

 constantly associated, rare or common species, may he of greater practical 

 utility and often of more exact value in correlation thun all the remainder 

 of a large fauna. For instance, after making sure that the beds of some 

 Appalachian exposure are of Cincinnatian age, I should depend more 

 on Orthorhyncula Unneyi in deciding that the layers containing this 

 fossil are strictly equivalent to the upper part of the Fairview limestone 

 at Cincinnati than on perhaps fifty other species that may be associated 



