Situated at approximately 18° North latitude and 65° West longitude, in the extreme northwest portion of the Caribbean island chain commonly referred to as the Leeward Islands, the Virgin Islands effectively represent the demarcation point between the Greater (to the West) and Lesser (to the East and South) Antilles. Mostly volcanic in origin, the one hundred or so islands, islets, cays, and rock outcroppings that form the Virgins are divided into three basic groups: the Spanish Virgin Islands, situated directly East of the United States Commonwealth of Puerto Rico, of which they are a part; the United States Virgin Islands (St. John, St. Thomas, Water Island and St. Croix), lying East of the Spanish Virgin Islands; and the British Virgin Islands, lying roughly to the North and Northeast of the U. S. Virgin Islands.