The boys made good progress down the coast of tropical South America.  They had heard much of Peru, and were surprised to see only a great strip of sand, lying like a desert, between the Pacific and the mountains.  Now and then a little stream, fed by the melting snows in the Andes, comes trailing out toward the sea, but it is usually smaller at its mouth than at its source for the reason that the precious water is utilized for irrigation purposes.  Wherever there is water crops grow luxuriantly.