[CentOS] chmod for directories only

I want to change the permissions on directories, recursively to:

drwxr-xr-x

but keep the files within the directory tree as:

-rw-r--r--

But I can't find an option for chmod to only affect directories.  I 
suppose it won't hurt (in this case) to set the x for the files, but I 
want some consistancy.