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a vs. y

actually both Ma and My (note not "yr" by y is the proper abbreviation)
are quite correct, but have very different meanings:

Ma = million years ago
My = million years

the distinction here is between ages (years ago, ergo Ma) and time
spans (years ergo My).

Therefore, the Morrison Fm is approximately 150 Ma, and represents ~ 6 My.
The usage Myr has been instituted by a number of scientific journals who
refuse to correct their editorial policies even though numerous geochronologists
have tried to correct them.  If everyone uses them correctly, perhaps the 
journals will have to change!

b, geochronologist

Bonnie Blackwell,                               bonn@qcvaxa.acc.qc.edu
Dept of Geology,                                (718) 997-3332
Queens College, City University of New York,    fax:  997-3349
Flushing, NY 11367-1597