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Re: geologic periods

> On Tue, 10 Jan 1995, Tom Holtz wrote:
> The Cretaceous is THE longest formal period in Earth's history.  It 
was 15
> million years longer than the whole of the Cenozoic.  Dinosaur 
> averaged only some 3-6 million years or so.

Tony wrote:
> At the level of geologic period, the Cambrian was longer by several 
> of years (approximately 90 vs approximately 75).  Obviously the 
> is a formal period of time and is the longest formal time increment.

Here is the most up to date (1989) details of the geological column 
and the lengths of the Periods in millions of years.

'Quaternary'  = 1.64
Neogene      = 21.7
Paleogene   = 41.7
Cretaceous = 81.0
Jurassic     = 62
Triassic      = 37
Permian     = 45
Carboniferous = 73 (Pennsylvanian = 33; Mississippian = 40)
Devonian      = 46
Silurian       = 31
Ordovician  = 71
Cambrian    = 60
Vendian      = 40

As for the Precambrian......it is not a period, but a time before the 
Cambrian.  The Proterozoic Eon is a subdivision of the Precambrian 
and is in turn subdivided into the Neo-, Meso- and Palaeoproterozoic 
Eras which last a total of 1,930 million years.  These are subdivided 
intoPeriods that can last for a couple of hundred million years or so 
each and, therefore, could be considered as being the longest 
Periods.  However, I presume that Tom was referring to the longest 
Period in the Phanerozoic Eon.


Neil Clark
Curator of Palaeontology
Hunterian Museum
University of Glasgow
email: NCLARK@museum.gla.ac.uk

Mountains are found in erogenous zones.
(Geological Howlers - ed. WDI Rolfe)