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originally； geological history was divided into four spans of time： primary； secondary；tertiary； and quaternary。 the system was too neat to last； and soon geologists werecontributing additional divisions while eliminating others。 primary and secondary fell out ofuse altogether； while quaternary was discarded by some but kept by others。 today onlytertiary remains as a mon designation everywhere； even though it no longer represents athird period of anything。
lyell； in his principles； introduced additional units known as epochs or series to cover theperiod since the age of the dinosaurs； among them pleistocene （鈥渕ost recent鈥潱弧liocene（鈥渕ore recent鈥潱弧iocene （鈥渕oderately recent鈥潱弧nd the rather endearingly vague oligocene（鈥渂ut a little recent鈥潱！yell originally intended to employ 鈥湣璼ynchronous鈥潯or his endings；giving us such crunchy designations as meiosynchronous and pleiosynchronous。 thereverend william whewell； an influential man； objected on etymological grounds andsuggested instead an 鈥湣璭ous鈥潯attern； producing meioneous； pleioneous； and so on。 the 鈥湣璫ene鈥潯erminations were thus something of a promise。
nowadays； and speaking very generally； geological time is divided first into four greatchunks known as eras： precambrian； paleozoic （from the greek meaning 鈥渙ld life鈥潱籱esozoic （鈥渕iddle life鈥潱弧nd cenozoic （鈥渞ecent life鈥潱！hese four eras are further dividedinto anywhere from a dozen to twenty subgroups； usually called periods though sometimesknown as systems。 most of these are also reasonably well known： cretaceous； jurassic；triassic； silurian； and so on。
1then e lyell鈥檚 epochs鈥攖he pleistocene； miocene； and so on鈥攚hich apply only to themost recent （but paleontologically busy） sixty…five million years； and finally we have a massof finer subdivisions known as stages or ages。 most of these are named； nearly alwaysawkwardly； after places： illinoian； desmoinesian； croixian； kimmeridgian； and so on in likevein。 altogether； according to john mcphee； these number in the 鈥渢ens of dozens。鈥�
fortunately； unless you take up geology as a career； you are unlikely ever to hear any of themagain。
further confusing the matter is that the stages or ages in north america have differentnames from the stages in europe and often only roughly intersect in time。 thus the northamerican cincinnatian stage mostly corresponds with the ashgillian stage in europe； plus atiny bit of the slightly earlier caradocian stage。
also； all this changes from textbook to textbook and from person to person； so that someauthorities describe seven recent epochs； while others are content with four。 in some books；too； you will find the tertiary and quaternary taken out and replaced by periods of differentlengths called the palaeogene and neogene。 others divide the precambrian into two eras； thevery ancient archean and the more recent proterozoic。 sometimes too you will see the termphanerozoic used to describe the span enpassing the cenozoic； mesozoic； and paleozoiceras。
moreover； all this applies only to units of time 。 rocks are divided into quite separate unitsknown as systems； series； and stages。 a distinction is also made between late and early（referring to time） and upper and lower （referring to layers of rock）。 it can all get terriblyconfusing to nonspecialists； but to a geologist these can be matters of passion。 鈥渋 have seengrown men glow incandescent with rage over this metaphorical millisecond in life鈥檚 history；鈥�
the british paleontologist richard fortey has written with regard to a long…running twentieth…century dispute over where the boundary lies between the cambrian and ordovician。
at least today we can bring some sophisticated dating techniques to the table。 for most ofthe nineteenth century geologists could draw on nothing more than the most hopefulguesswork。 the frustrating position then was that although they could place the various rocksand fossils in order by age； they had no idea how long any of those ages were。 whenbuckland speculated on the antiquity of an ichthyosaurus skeleton he could do no better thansuggest that it had lived somewhere between 鈥渢en thousand； or more than ten thousand timesten thousand鈥潯ears earlier。
although there was no reliable way of dating periods； there was no shortage of peoplewilling to try。 the most well known early attempt was in 1650 when archbishop jamesussher of the church of ireland made a careful study of the bible and other historical sourcesand concluded； in a hefty tome called annals of the old testament ； that the earth had been1there will be no testing here； but if you are ever required to memorize them you might wish to remember johnwilfords helpful advice to think of the eras （precambrian； paleozoic； mesozoic； an（ cenozoic） as seasons in ayear and the periods （permian； triassic jurassic； etc。） as the months。
created at midday on october 23； 4004b。c。 ； an assertion that has amused historians andtextbook writers ever since。
2there is a persistent myth； incidentally鈥攁nd one propounded in many serious books鈥攖hatussher鈥檚 views dominated scientific beliefs well into the nineteenth century； and that it waslyell who put everyone straight。 stephen jay gould； in time鈥檚 arrow； cites as a typicalexample this sentence from a popular book of the 1980s： 鈥渦ntil lyell published his book；most thinking people accepted the idea that the earth was young。鈥潯n fact； no。 as martin j。 s。
rudwick puts it； 鈥渘o geologist of any nationality whose work was taken seriously by othergeologists advocated a timescale confined within the limits of a literalistic exegesis ofgenesis。鈥潯ven the reverend buckland； as pious a soul as the nineteenth century produced；noted that nowhere did the bible suggest that god made heaven and earth on the first day；but merely 鈥渋n the beginning。鈥潯hat beginning； he reasoned； may have lasted 鈥渕illions uponmillions of years。鈥潯veryone agreed that the earth was ancient。 the question was simply howancient。
one of the better early attempts at dating the planet came from the ever…reliable edmondhalley； who in 1715 suggested that if you divided the total amount of salt in the world鈥檚 seasby the amount added each year； you would get the number of years that the oceans had beenin existence； which would give you a rough idea of earth鈥檚 age。 the logic was appealing； butunfortunately no one knew how much salt was in the sea or by how much it increased eachyear； which rendered the experiment impracticable。
the first attempt at measurement that could be called remotely scientific was made by thefrenchman georges…louis leclerc； te de buffon； in the 1770s。 it had long been knownthat the earth radiated appreciable amounts of heat鈥攖hat was apparent to anyone who wentdown a coal mine鈥攂ut there wasn鈥檛 any way of estimating the rate of dissipation。 buffon鈥檚experiment consisted of heating spheres until they glowed white hot and then estimating therate of heat loss by touching them （presumably very lightly at first） as they cooled。 from thishe guessed the earth鈥檚 age to be somewhere between 75；000 and 168；000 years old。 this wasof course a wild underestimate； but a radical notion nonetheless； a