The soil is fertile and may be divided into three classes. First, the red soil, generally found in the mountains; second, the black soil, containing much humus, and third, the coral sand soil of the coast plains. The black has proved to be the best for sugar cane, although excellent yields have been obtained from the coast lands. Notwithstanding the fact that both soil and climate are well suited to cane cultivation, the extension of the industry is checked by the formation of the country. The hilly character of the island and the comparatively limited transportation facilities do not admit of cane being grown in the interior. All of the modern plantations are near the coast, where sugar can be easily transported to steamers. In the north cane may be raised without irrigation, but in the south, where the greater part of the crop is produced, irrigation is necessary. About 400,000 acres of the surface of Porto Rico are under cultivation and half of this area is devoted to sugar. The cane grows chiefly on the rich alluvial lowlands along the coast. On the southern seaboard there is plenty of good land that has never been planted, but to make this available for agriculture means the construction of costly irrigation works with extensive aqueducts and much tunneling. Porto Rico was discovered by Columbus in 1493. The name Puerto Rico dates from 1521, when gold was found in the island. The natives suffered cruelly at the hands of their conquerors up to the year 1544, when they were set free by the order of King Charles I of Spain.[43] Out of a population originally estimated at 600,000 only a few hundred remained. Great numbers had died and many had fled to Mexico and Peru, but the Spaniards soon filled their places with African negroes. For three centuries the island was harried by the English, attacked by pirates and freebooters, torn by rebellion among the slaves or terrorized by the fleets of the Dutch, who were intent upon its conquest, so that Spain was kept constantly on her guard to prevent her colony from being wrested from her. This state of affairs naturally did not encourage population. The country could not meet the cost of its administration and the deficit had to be paid out of the revenues of Mexico. Up to 1778 only Spaniards were allowed to settle there, but after that date the privilege was granted to people of other countries, provided they were of the Roman Catholic faith, and in 1815 all restrictions were removed. Foreigners were welcomed to the island and many inducements offered them. Trade with the United States was permitted in 1815, but only in Spanish ships. This liberal policy brought many planters from the British and French islands, and as they had experience, capital and slaves, they did much to develop the resources of the colony. Subsequently many refugees came from Haiti, Santo Domingo and Venezuela. Slavery in Spanish possessions was abolished in 1873 and 34,000 slaves in Porto Rico received their freedom.