Spread before you a map of South America. Fix your eye on the point of confluence between two of its great rivers-the Salado, which runs south-easterly from the Andes mountains, and the Parana coming from the north; carry your glance up the former to the town of Salta, in the ancient province of Tucuman; do likewise with the latter to the point where it espouses the Paraguay; then up this to the Brazilian frontier fort of Coimbra; finally draw a line from the fort to the aforementioned town-a line slightly curved with its convexity towards the Cordillera of the Andes-and you will thus have traced a boundary embracing one of the least known, yet most interesting, tracts of territory in either continent of America, or, for that matter, in the world.