506 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



of the succession of the lithological units and of the fossil faunas and 

 floras entombed in them. 



(h) Geographic continuity. — This is established by tracing the beds 

 by actual outcrop from point to point, or by reasonable inference from 

 data obtained from deep wells in undisturbed areas. 



Under the second head we place correlation by 



(a) Similarity of organic remains. 



(6) Similarity of lithologic characters, including similarity in genesis 

 or homogeny. 



(c) Evidence of deformative movements of the lithosphere. 



(d) Evidence of gradual submergence, usually indicated by strati- 

 graphic overlaps and ascribed to various causes, such as sea-filling, possi- 

 ble equatorial heaping of waters, and slow adjustment to gravitational 

 stresses. 



In the further discussion of these methods those of the first group, 

 namely, superposition and geographic continuity, might be passed over as 

 too obvious to require detailed explanation. All will understand that 

 relations established by superposition go no farther than the facts brought 

 out by individual sections. These may express a complete sedimentary 

 record for the part of the stratigraphic column shown in them; or they 

 may be more or less incomplete, the locally absent elements being estab- 

 lished by comparison with other sections in which they are present. The 

 completion of the record, or rather of so much of it as is accessible, may 

 involve all the other methods. As to the areal continuity of a lithologic 

 unit, this of itself does not establish contemporaneity of the geographic- 

 ally separated parts of the unit. This is true especially of the overlapping 

 initial deposits (usually sandstones) of an advancing sea; but the range of 

 time included in known examples is, not as great as is thought by some. 

 As the practical application of this method of correlation, as also that 

 based on stratigraphic overlap, is closely associated with and must be 

 checked by certain deductive methods, its further discussion may well be 

 made incidental to matter more strictly referred to under the headings 

 "Correlation by lithological similarity^^ and "Correlation by evidence of 

 diastrophie movements." 



CORRELATION BY MEANS OF ORGANIC REMAINS 



General discussion. — Correlation by means of fossils is universally ac- 

 cepted. It is also quite generally believed that fossil evidence is relatively 

 the most competent of the several classes of evidence now employed in cor- 

 relation. That the rank of organic evidence is of the highest can not be 



