Library is not writable

I have this issue during package installation in R version 3.0.2 (2013-09-25) on an Ubuntu machine:

install.packages("randomForest")Installing package into ‘/usr/local/lib/R/site-library’(as ‘lib’ is unspecified)Warning in install.packages :'lib = "/usr/local/lib/R/site-library"' is not writable

How to resolve it?