He knew that at the eastern rim of Bolivia there was a series of high mountain ranges which would protect him from the drifts blowing over from the Atlantic--Serre Geral, Serre Paxecis, Serre Aguapehy--and he reasoned that he could make better speed under the lee of these elevations.  So he swept down the valley of the Beni until it joined with the Madeira, crossed a line of hills, and made for the Serre Geral range, something under a hundred miles away.