Automate tick max and min in faceted ggplot

I am trying to just mark the max and min of each x-axis in a faceted ggplot. I have several facets with different x scales and the same y scale, and the x axis tick labels overlap each other. Rather than having to manually determine the limits and breaks for each facet x axis, I am looking for a way to just label the min and max values for each.

Code using example data of the CO2 dataset (see ?CO2):

CO2$num <- 1:nrow(CO2)
library(reshape2)
CO2.melt <- melt(CO2,
                 id.var=c("Type",
                          "Plant",
                          "Treatment",
                          "num"))
CO2.melt <- CO2.melt[order(CO2.melt$num),]

library(ggplot2)
ggplot(CO2.melt, 
       aes(x = value, 
           y = num)) +
  geom_path(aes(color = Treatment)) +
  facet_wrap( ~ variable, scales = "free_x",nrow=1)

Purpose is to replicate well log displays such as this one.