562 - Heredify and Evolution 



Fig. 29-9. Layer after layer of stratified rock, exposed in the Grand Canyon of the Colorado River. (Union 

 Pacific Railroad Photograph.) 



is far from complete. In fact, it is surprising 

 that so much has been preserved and dis- 

 covered. Since the time of Darwin — who 

 emphasized the meagerness of fossil data — 

 paleontologists have advanced great lengths 

 toward the completion of the record (Fig. 

 29-10), especially with reference to species 

 that became extinct during the more recent 

 periods of geological time. 



Geological Strata and Their Relative Ages. 

 The layers, or strata, of sedimentary rock, 

 because of the nature of their origin, occur 

 in the sequence of their deposition, such that 

 all newer (later) strata are superimposed 

 upon the older (earlier) layers. Consequently 

 the deeper the stratum, the older its content 

 of fossils. However, the problem of determin- 

 ing the relative ages of the various geological 



strata is not always so very simple, because 

 not all strata are to be found in any one lo- 

 cality. Some land areas were exposed at times 

 when others were submerged, so that great 

 gaps may be present in the strata of any 

 given region. Also some areas experienced 

 resubmergence after many centuries of ex- 

 posure — during which erosion carried away 

 many of the upper strata. Therefore, in some 

 cases, relatively recent strata are found super- 

 imposed directly upon very ancient layers. 

 But despite the gaps, the older strata in any 

 one locality are always found to be deeper 

 than the newer layers, except in rare cases 

 where there has been a demonstrable folding 

 of the crust of the earth and an inversion of 

 the strata. And once the relative age of a 

 certain stratum has been determined on 



