PERIOD

in geology, the basic unit of the geologic time scale; during these spans of time specific systems of rocks were formed. Originally, the sequential nature of defining periods was a relative one, originating from the superposition of corresponding stratigraphic sequences and the evidence derived from paleontological studies. With the advent of radiometric dating methods, absolute ages for various periods can be determined. in music, unit of melodic organization made up of several related consecutive phrases. Often the period has two phrases, typically of 8 or, in some cases, 16 measures each. The first phrase, called the antecedent, ends on a semi- or half-cadence and thus avoids any sense of finality; the second comes to a full cadential stop on the tonic, unless a modulation has occurred, in which case the full cadence confirms the new key. Periodic structure is primarily a function of homophonic and especially diatonic music, in which the kind of sectionalization associated with the musical period operates at all structural levels. Polyphonic and monophonic textures, on the other hand, thrive on nonperiodicity, or aperiodicity, precisely because the resulting lack of clearly defined intersections generates the continuous flow of melodic energy that is the essence of music thus conceived.