As students of the earth, we have looked largely to the sea for records of earth history. This predominant interest in an environment remote from our continental habitat has been forced upon us by the prevalence of marine deposits in regions where geology and paleontology have been most fully studied. As typically terrestrial members of the contemporary mammalian fauna, we might be expected to concern ourselves largely with the continents and land life of the past. But in most rocks, the record is not sufficiently complete to chronicle the events and populations above sea level. Not choice but necessity has dictated our dependence upon the sands of the sea to tell us the story of the earth.
Unlike much of the record of earlier eras, deposits of the Cenozoic permit us to extend our study of earth history far inland from ancient shores. Vulcanism during the Tertiary provided conditions favorable . . .