Geologic understanding depends on interpreting earth history using relative time scales derived from the study of positional relationships of rock units and mineral bodies. Age determinations of specific rocks and events are used to calibrate a relative time scale. The fundamental problem in geochronology is correlation. Scientists date fossils, rocks, minerals, and remanent magnetism by inferring their position in a geologic succession and correlating them to a relative time scale. The only real evidence for an interval of time is relative position which is extended by hypotheses of correlation. Fossils and radiometric specimens, by their succession, may indicate the direction of time because of evolutionary changes or relative decay of elements. In magnetostra- tigraphy, apparent polar wandering may similarly indicate time direction but successive paleomag- netic reversals show no such direction and must therefore be "dated" by some other method before accurate magnetic correlation can be used.
The biochronological time scale is an empirical artifact that has grown haphazardly over the last 200 years in areas determined by extraneous factors in regard to development of geologic concepts. The pitfalls of biochronology are known, but the vast and increasing body of empirical knowledge continues to reduce major possibilities of error. Although paleontology provides the "empirical basis for evolutionary theory" it must not be forgotten that the concept of evolution is derived from superposition and correlation. Within the Phanerozoic, accuracy of biochronology does not decrease with distance in time, although application may be strongly facies controlled. In radio- metric dating, the percentage error may remain constant, therefore decreasing resolution with increasing age. The difficulty in applying radiometric systems is in knowing what event has been dated; the materials may have undergone successive changes subsequent to their formation. Similar considerations apply to paleomagnetic evidence.
Since the first relative time scales were constructed, new information and insights have emerged from both continental areas and the ocean floor. New methods of correlation now allow hope for an accurate time scale unifying biochronology, radiometry, and magnetostratigraphy. Multiple systems must be used within each major discipline. Thus correlation of many animal or plant groups may achieve greater precision in biochronology than single zonal indices. Similarly, several independent radiometric dating systems may increase confidence or help distinguish successive events that may be detectable by such methods. The most important single task of geochronology is to construct a unified time scale using the strengths of all methods and to test the time scale repeatedly by stratigraphic reproducibility. The potential accuracy of a unified time scale hasscarcely been realized.