incremental tar question

[Reed L. O'Brien]

FreeBSD 5.2.1

I am using the folowing to backup saturday

tar -jcf etc.bz2 /etc /usr/local/etc

in /bak/saturday.

I would like to do an incremental on monday (et al) in /bak/monday (et al)

tar -cf etc.bz2 /etc/ /usr/local/etc/ -g /bak/saturday/etc.bz2

Is this the correct format? I cannot get it to work. I can get it to 
leave a big enough bz2 file in /bak/monday but I can't untar it. though 
I can the initial full backup from saturday.

Do I need to copy the bz2 archive to /bak/monday?
Do I need a different command?
Does -g not support bzip2?

ideas?
FreeBSD 5.2.1
TIA
reed