[BioC] downloading a conservation track with rtracklayer on several chromosome seqs

[Robert Castelo]

dear list,

i'd like to know how can i download the conservation track for several
genomic ranges belonging to different chromosome sequences as the
example below seems only to retrieve this information for 1 of the
chromosomes, (the first one on the list):

library(rtracklayer) 

session <- browserSession("UCSC")

genome(session) <- "hg18" 

track(session, "Conservation",
GenomicRanges(start=c(27099139,52708461,32260488),end=c(27102150,52710874,32265941),chrom=c("chr7","chr12","chr6"), genome="hg18"))
UCSC track 'Primate Cons'
UCSCData: 3012 ranges by 1 columns
columns(1): score
sequences(1): chr7

thanks!
robert.

sessionInfo()
R version 2.9.1 (2009-06-26) 
x86_64-unknown-linux-gnu 

locale:
C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods
base     

other attached packages:
[1] rtracklayer_1.4.1 RCurl_0.98-1      bitops_1.0-4.1   

loaded via a namespace (and not attached):
[1] BSgenome_1.12.2   Biobase_2.4.1     Biostrings_2.12.7
IRanges_1.2.3    
[5] XML_2.5-3