    Sentence 1: The preserice’‘or. absence of-what can be used.  to. determirie the felative age of the formations in’ which  ‘they are found?    Sentence.2: As ‘organisms exist’ at the same time: period.   : throughout’ the. world, their’ presence.- or. (sometimes)  bsencé. may be’ used: to: provide ‘a relative age of. the’  formations in which ‘they are found.