[R-sig-Geo] Merge rasters with different extent

[Thiago Veloso]

 Dear list,

 I have two rasters (r1 and r2). r1 has larger extent; r2 is smaller and its extents fall totally within r1. Here are the rasters:

> r1

class       : RasterLayer 
dimensions  : 360, 720, 259200  (nrow, ncol, ncell)
resolution  : 0.5, 0.5  (x, y)
extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0 
values      : in memory
layer name  : layer 
min value   : 0 
max value   : 0.055233 

> r2

class       : RasterLayer 
dimensions  : 47, 65, 3055  (nrow, ncol, ncell)
resolution  : 0.5, 0.5  (x, y)
extent      : -74.5, -42, -18, 5.5  (xmin, xmax, ymin, ymax)
coord. ref. : NA 
values      : in memory
layer name  : layer 
min value   : 0 
max value   : 1 

 What I would like to be able to do is merge both rasters, but keeping r2 values when cells overlap. In other words, replace r1 values with r2 values only when cells overlap. Following older discussions, I have tried "merge" command without sucess. Neither "r3 <- merge(r1,r2)" nor "r3 <- merge(r2,r1)" yield what I pursue. 

 Any ideas?

 Many thanks in advance,

 Thiago.