[R] gridlines in contour plot

[FMH]

Dear All,

Someone suggesting me to use the filled.contour function to plot the image together with the color index, and an example from the help menu is show below. 
 
#################################################################################
require(grDevices) # for colours
filled.contour(volcano, color = terrain.colors, asp = 1)# simple
x <- 10*1:nrow(volcano)
y <- 10*1:ncol(volcano)
filled.contour(x, y, volcano, color = terrain.colors, 
    plot.title = title(main = "The Topography of Maunga Whau",
    xlab = "Meters North", ylab = "Meters West"),
    plot.axes = { axis(1, seq(100, 800, by = 100))
                  axis(2, seq(100, 600, by = 100)) },
    key.title = title(main="Height\n(meters)"),
    key.axes = axis(4, seq(90, 190, by = 10)))# maybe also asp=1
#################################################################################
 
I am still trying to draw several gridlines with 10m gaps, which might represent the height of the mountain on the image but still can't manage it. 
 
Could someone please give some hints?
 
Thank you
Fir