Show history of earth divided into five eras seven ages and 22 periods the divisions of time established by geologists are based upon the formations of strata and the ad vents ot of different forms of animal life the history of the earth Is divided into five eras seven ages 22 periods and the last two periods are subdivided into seven epochs these divisions proceeding from the tha fifth downward to the first states a writer in the cle cleveland ve land plain dealer are as follows fifth era age of man human period and recent epoch fourth cenozoic Ceno zole era age of mammals embracing the quater nary period which comprehends the terrace champlain Champl aln and glacial epochs and the tertiary period which comprehends the pliocene nl pi ocene and eocene epochs third mesozoic or middle era the age of reptiles the cretaci cretaceous ea us jurassic and triassic lassic tr periods second paleozoic err the car coniferous boni ferous erous age or age of ac and amphibians the devonian age or age of fishes the silurian age or age of invertebrates or mollusks the names of the fourteen periods into which these ages are divided are not in common use first firs t or era the age and the huronian Hu ronlan and laurentian Laurent lan periods the term Is restricted by the united states geological survey and most american geologists to the earlier portion of the precambrian pre cambrian represented in the record chiefly b by y metamorphosed sed igneous rocks but bu t sub gub by metamorphosed sedimentary beds the rocks contain carbonaceous material iron ores lime stones etc which probably indicate the existence of life but no fossils have been found the duration of the era during which the rocks were formed was wai very great possibly exceed exceeding ing all subsequent time |