Any of these work for you?
## scatter
plot(quakes$long, quakes$lat)
library(hexbin)
plot(hexbin(quakes$long, quakes$lat))
library(ggplot2)
ggplot(quakes, aes(x = long, y = lat)) + stat_density2d(aes(alpha = ..level..), geom = "polygon")
ggplot(quakes, aes(x = long, y = lat)) + stat_density2d(aes(fill = ..density..), geom = "tile", contour = FALSE)
library(MASS)
filled.contour(kde2d(quakes$long, quakes$lat))