▁Cry ogen ian : ▁The ▁middle ▁period ▁in ▁the ▁Ne opro ter oz o ic ▁Era : ▁- ▁Ma . ▁ ▁Ton ian : ▁the ▁earliest ▁period ▁of ▁the ▁Ne opro ter oz o ic ▁Era : ▁- ▁Ma . ▁ ▁Mes opro ter oz o ic : ▁the ▁middle ▁era ▁of ▁the ▁Pro ter oz o ic ▁E on , ▁- ▁Ma . ▁Cor respond s ▁to ▁" P rec amb rian ▁Y " ▁rocks ▁of ▁older ▁North ▁American ▁ge ology . ▁ ▁Pale opro ter oz o ic : ▁oldest ▁era ▁of ▁the ▁Pro ter oz o ic ▁E on , ▁- ▁Ma . ▁Cor respond s ▁to ▁" P rec amb rian ▁X " ▁rocks ▁of ▁older ▁North ▁American ▁ge ology . ▁ ▁Ar che an ▁E on : ▁- ▁Ma . ▁ ▁H ade an ▁E on : ▁– ▁Ma . ▁This ▁term ▁was ▁intended ▁originally ▁to ▁cover ▁the ▁time ▁before ▁any ▁preserved ▁rocks ▁were ▁depos ited , ▁although ▁some ▁z ir con ▁cry st als ▁from ▁about ▁ 4 4 0 0 ▁Ma ▁demonstrate ▁the ▁existence ▁of ▁cr ust ▁in ▁the ▁H ade an ▁E on . ▁Other ▁records ▁from ▁H ade an ▁time ▁come ▁from ▁the ▁moon ▁and ▁meteor ites . ▁It ▁has ▁been ▁proposed ▁that ▁the ▁P rec amb rian ▁should ▁be ▁divided ▁into ▁e ons ▁and ▁er as ▁that ▁reflect ▁stages ▁of ▁planet ary ▁evolution , ▁rather ▁than ▁the ▁current ▁scheme ▁based ▁upon ▁numerical ▁ages . ▁Such ▁a ▁system ▁could ▁rely ▁on ▁events ▁in ▁the