Our boat floated on , between walls of forest too thick to allow us a view of the land we were passing through , though we knew from the map that our river must from time to time be passing through chains of hills which crossed the jungle plains . Nowhere did we find a place where we could have landed: where the jungle did not actually spread right down into the river , banks of soft mud prevented us going ashore . In any case , what would we have sailed by landing ? The country was full of snakes and other dangerous creatures , and the jungle so thick that one would be able to move forward only slowly , cutting one’s way with knives the whole way . So we stayed in the boat , hoping that when we reached the sea , a friendly fisherman would pick us up and take us to civilization .