Obsolete words/dating from 1660-WHY??

[Ann Rogers <ROGERSA01@TEN-NASH.TEN.K12.TN.US>]

I have an English teacher doing Etymology research with her class.  The
question has come up as to why "obsolete level of usage" means words not in
print since 1660.  What happened in 1660 to cause this, we suspect it is do
to the restoration of the monarchy in England.  How did the Restoration
affect words being in print?  Any information would be greatly appreciated.