Treaty of Concordia
The Treaty of Concordia (or the Partition Treaty of 1648) was signed on March 23, 1648 between the French and the Dutch. The signing took place atop Mount Concordia. Based on the terms of the agreement, the island of Saint Martin was to be divided between the French Kingdom and the Dutch Republic and that the peoples of St.Martin shall coexist in a cooperative manner. However, France and the Netherlands would continue to dispute over the ownership of the island until 1817 when the borders of the island were finally set.
Text of the Treaty of Concordia (1648)
See here the Text of the Treaty of Concordia (Wikisource : English translation of the original text in French).
Validity of the treaty of March 23, 1648
Questions as to the validity of the Treaty of 1648 have arisen several times in the past, and continue to cause issues today. Research was supposed to be undertaken by the French Ministry of Foreign Affairs about it.
At the time of the Treaty, agreements signed between representatives of the monarch had to be registered at the King's Council. This essential formality was never performed, but it seems that on many occasions French civil law recognizes the validity of this Agreement.