Unfortunately, there is no simple way to do that since chmod doesn't have a "directory only" option.
You may be interested to read this:
and the reply!
My idea would be to use the -p option in ls and then grep the / at the end of directories like that:
ls -p | grep /
to get a list of the directories, put this into a file and write a shell script to chmod the element in the list. There probably is a more efficient way to do it, but that's what I would do!
Hope this helps!