. Principles of modern biology. Biology. 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
. Principles of modern biology. Biology. 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original
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Keywords: ., bookcentury1900, bookcollectionbiodiversity, booksubjectbiology
