包'rjson'的R - twitteR软件包下载失败了

我正在尝试一些数据挖掘并尝试从Twitter检索数据.

当我尝试安装包'twitteR'时,我收到以下警告:

Warning in install.packages :
  download of package ‘rjson’ failed

但它加载了其余的包.然后,当我尝试调用库时:

> library(twitteR)
Loading required package: ROAuth
Loading required package: RCurl
Loading required package: bitops

Attaching package: ‘RCurl’

The following object is masked from ‘package:tm.plugin.webmining’:

    getURL

Loading required package: digest
Error: package ‘rjson’ required by ‘twitteR’ could not be found

如果它最初无法下载'rjson'软件包,这是有道理的.

当我尝试单独安装'rjson'软件包时,我得到一个熟悉的错误:

> install.packages("rjson")
trying URL 'http://cran.rstudio.com/bin/macosx/contrib/3.0/rjson_0.2.13.tgz'
Warning in install.packages :
  cannot open: HTTP status was '404 Not Found'
Error in download.file(url, destfile, method, mode = "wb", ...) : 
  cannot open URL 'http://cran.rstudio.com/bin/macosx/contrib/3.0/rjson_0.2.13.tgz'
Warning in install.packages :
  download of package ‘rjson’ failed

我不熟悉这些错误的疑难解答.很感谢任何形式的帮助.