calculate the mean for each column of a matrix in R

I am working on R in R studio. I need to calculate the mean for each column of a data frame.

 cluster1  // 5 by 4 data frame
 mean(cluster1) // 

I got :

  Warning message:
  In mean.default(cluster1) :
  argument is not numeric or logical: returning NA

But I can use

  mean(cluster1[[1]])

to get the mean of the first column.

How to get means for all columns ?

Any help would be appreciated.