R typically installs libgfortran so packages can link to it, but for some reason it wasn't linked properly for me.
ln -s $(R RHOME)/lib/libgfortran.2.dylib $(R RHOME)/lib/libgfortran.dylib
Also for some reason R wasn't able to find its own shared library directory, so download geiger
curl -O https://dl.dropbox.com/u/8859543/geiger_1.4-4.666.tar.gz
tar xvf geiger_1.4-4.666.tar.gz
rm -rf geiger/src/._* # remove mac os garbage
Edit geiger/src/Makevars
(fixing a typo along the way) so it looks like:
PKG_CXXFLAGS = `$(R_HOME)/bin/Rscript -e "Rcpp:::CxxFlags()"`
PKG_LIBS = `$(R_HOME)/bin/Rscript -e "Rcpp:::LdFlags()"` ${BLAS_LIBS} ${FLIBS} -L${R_HOME}/lib
Then try
R CMD INSTALL geiger/
You can also use the compiled binary package in R
curl -O https://raw.github.com/gist/4211495/geiger_1.4-4.666.tgz
R CMD INSTALL geiger_1.4-4.666.tgz