In the Times of Scrambles in the Land of the Amazons - The Scramble for the Amazon and the Lost Paradise of Euclides da Cunha - page 77

betweentheNewWorldholdingsoftheSpanishandPortugueseempires.Theannulment
of this foundational division was a pathbreaking precedent and served to open up the
Amazon to many contenders, since the territorial “rights” of Spain no longer counted
in substantial territories and Brazil barely had a presence there. Next, the treaty ratified
the use of the Roman law concept of uti possedetis —he who has, keeps. The use of this
principle in border negotiations would inform the substance of South American disputes
forthenext150years.Further,thebordersweretofollowclearlydiscerniblelandmarks,
in this case rivers, that at least made the borders far more obvious—but whether a given
river was in fact the indicated border became a problem in other frontier cases. A final
elementwastheuseofboundarysurveystoconsolidatetheshiftsinterritorialdominion.
The Madrid Treaty included a clause that underpinned Amazonian contention later: a
straight parallel line would be drawn from the middle of the Rio Madeira (in this case
taken to be just above the extensive falls) to the (unknown) headwaters of the Rio Jav-
ari at 6.53 degrees south latitude; this would constitute the western boundary between
the Portuguese and Spanish colonies in their (unexplored) northern reaches. This clause
wouldinflame theScramble acenturylater,asthisprinciple wasratified intheTreatyof
San Idelfonso.