Relative dating principles of rocks
Correlation may be difficult or erroneous if several different ash eruptions occurred, and a layer deposited in one is correlated with that from another.Even then, the correlation may be justified if the two ash deposits represent the same volcanic episode.Presumably if all the world’s outcrops were integrated, sediments representing all of geologic time would be available for examination.
On the practical side, an index fossil should be distinctive in appearance so as to prevent misidentification, and it should be cosmopolitan both as to geography and as to rock type.
By this technique it is possible to detect depth variations in electrical resistivity, self-potential, and gamma-ray emission rate and to interpret such data in terms of continuity of the layering between holes.
Subsurface structures can thus be defined by the correlation of such layer has four other advantages for purposes of correlation: it was laid down in an instant of geologic time; it settles out over tremendous areas; it permits physical correlation between contrasting sedimentary environments; and unaltered mineral crystals that permit radiometric measurements of absolute age often are present.
Correlating two separated outcrops means establishing that they share certain characteristics indicative of contemporary formation.
The most useful indication of time equivalence is similar fossil content, provided of course that such remains are present.