We are accustomed to think of Spanish commerce with the Indies as being made solely by great fleets which sailed yearly from Seville or Cadiz to Mexico and the Isthmus of Darien. There were, however, always exceptions to this rule. When, as sometimes happened, the Flota did not sail, two ships of 600 or 700 tons were sent by the King of Spain to Vera Cruz to carry the quicksilver necessary for the mines. The metal was divided between New Spain and Peru by the viceroy at Mexico, who sent _via_ Gautemala the portion intended for the south. These ships, called "azogues," carried from 2000 to 2500 quintals[28] of silver, and sometimes convoyed six or seven merchant vessels. From time to time an isolated ship was also allowed to sail from Spain to Caracas with licence from the Council of the Indies and the _Contratacion_, paying the king a duty of five ducats on the ton. It was called the "register of Caracas," took the same route as the galleons, and returned with one of the fleets from Havana. Similar vessels traded at Maracaibo, in Porto Rico and at San Domingo, at Havana and Matanzas in Cuba and at Truxillo and Campeache.[29] There was always, moreover, a special traffic with Buenos Ayres. This port was opened to a limited trade in negroes in 1595. In 1602 permission was given to the inhabitants of La Plata to export for six years the products of their lands to other Spanish possessions, in exchange for goods of which they had need; and when in 1616 the colonists demanded an indefinite renewal of this privilege, the sop thrown to them was the bare right of trade to the amount of 100 tons every three years. Later in the century the Council of the Indies extended the period to five years, so as not to prejudice the trade of the galleons.[30]