library("dplyr")
geometric.mean <- function(x){
  prop <- c(na.omit(x / dplyr::lag(x)))    
  return(exp(mean(log(prop))))
}

x <- c(1000, 1000*1.1, 1100*1.2, 1320*0.85)
geometric.mean(x)

>   geometric.mean(x)
[1] 1.039117