METHODS OF CORRELATION 1133 



tant from the shore than the other, may experience a sympathetic 

 change in sedimentation, when simultaneously affected by an oscilla- 

 tory movement. Thus, when in the near-shore region muds change 

 upward into sands, and still higher into muds again, the corre- 

 sponding change in the more distant region may be from lime- 

 stones to terrigenous muds and higher still to limestones again. 

 Such sympathetic changes seem to have taken place between the 

 New York and Michigan Hamilton deposits. 



4. Organic contents. 



Correlation by organic contents, or PaUeontologic correlation, 

 has been found to be the most reliable method, far surpassing in 

 importance any other single method. Nevertheless, there are many 

 pitfalls which must be guarded against, and the sources of error 

 must be recognized and taken into account. 



(a) Index fossils. Index fossils have already been defined 

 as species characteristic of definite geologic horizons, and typically 

 occurring only in beds of that horizon (page 1094). Index fossils 

 in order to be efficacious must be of limited vertical but wide hori- 

 zontal distribution. Thus the brachiopod Hxpothyris cuboides 

 characterizes a certain zone in the Upper Devonic of America, 

 Europe and Asia, while the Goniatite Manticoceras intuinescens 

 likewise characterizes late Devonic rocks throughout much of the 

 northern hemisphere. Similarly, Spirifer disjunctus has a limited 

 vertical range, combined with a wide horizontal one, being char- 

 acteristic of the Upper Devonic of many countries. Locally, the 

 type may transgress the normal vertical range, as in the case of 

 the last-mentioned species, which passes up into Lower Mississippic 

 beds in eastern North America, or, as in the case of Tropidoleptus 

 carinatus, a widespread index fossil of the Mid-Devonic, but which 

 locally passes into the Upper Devonic. The best index fossils are 

 those which are capable of wide distribution, and remains of whiclV 

 will occur in regions where the organisms may never have lived, 

 and in sediments which may differ widely from those forming the 

 normal facies of sea bottom for the type in cjuestion. As pointed 

 out in an earlier chapter, epiplanktonic and pseudoplanktonic forms 

 are most likely to produce such index fossils. The shell of Spirula, 

 a dibranchiate cephalopod of the modern fauna, illustrates the wide 

 distribution possible by flotation, though the animal has been found 

 to occur only in a few localities in deep water. Epiplanktonic Hy- 

 drozoa and Bryozoa likewise suffer a wide distribution through 

 the flotation of their host. 



