Re: geologic periods

[NEIL CLARK <NCLARK@museum.gla.ac.uk>]

> 
> 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 
species
> averaged only some 3-6 million years or so.
> 

Tony wrote:
> 
> At the level of geologic period, the Cambrian was longer by several 
millions
> of years (approximately 90 vs approximately 75).  Obviously the 
Precambrian
> 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


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)