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y 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’s epochs—the pleistocene； miocene； and so on—which 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 “tens of dozens。”
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。 “i have seengrown men glow incandescent with rage over this metaphorical millisecond in life’s history；”
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 “ten thousand； or more than ten thousand timesten thousand” years 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—and one propounded in many serious books—thatussher’s views dominated scientific beliefs well into the nineteenth century； and that it waslyell who put everyone straight。 stephen jay gould； in time’s arrow； cites as a typicalexample this sentence from a popular book of the 1980s： “until lyell published his book；most thinking people accepted the idea that the earth was young。” in fact； no。 as martin j。 s。
rudwick puts it； “no geologist of any nationality whose work was taken seriously by othergeologists advocated a timescale confined within the limits of a literalistic exegesis ofgenesis。” even 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 “in the beginning。” that beginning； he reasoned； may have lasted “millions uponmillions of years。” everyone 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’s 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’s 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—that was apparent to anyone who wentdown a coal mine—but there wasn’t any way of estimating the rate of dissipation。 buffon’sexperiment 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’s age to be somewhere between 75；000 and 168；000 years old。 this wasof course a wild underestimate； but a radical notion nonetheless； and buffon found himselfthreatened with exmunication for expressing it。 a practical man； he apologized at oncefor his thoughtless heresy； then cheerfully repeated the assertions throughout his subsequentwritings。
by the middle of the nineteenth century most learned people thought the earth was at leasta few million years old； perhaps even some tens of millions of years old； but probably notmore than that。 so it came as a surprise when； in 1859 in on the origin of species ； charlesdarwin announced that the geological processes that created the weald； an area of southernengland stretching across kent； surrey； and sussex； had taken； by his calculations；306；662；400 years to plete。 the assertion was remarkable partly for being so arrestinglyspecific but even more for flying in the face of accepted wisdom about the age of the earth。
3itproved so contentious that darwin withdrew it from the third edition of the book。 the2although virtually all books find a space for him； there is a striking variability in the details associated withussher。 some books say he made his pronouncement in 1650； others in 1654； still others in 1664。 many cite thedate of earths reputed beginning as october 26。 at least one book of note spells his name 〃usher。〃 the matter isinterestingly surveyed in stephen jay goulds eight little piggies。
3darwin loved an exact number。 in a later work； he announced that the number of worms to be found in anaverage acre of english country soil was 53；767。
prob