The western field and the western part of the central field were first worked in 1899-1900. The Salt Lake field, controlled by the Salt Lake Oil Company, near Rancho de Brea, W.S.W. of the city, first became important in 1902 and in 1907 it was the most valuable field in California, S. of Santa Barbara county, and the value of its product was $1,749,980. In 1905 the value of petroleum refined in Los Angeles was $461,281. Land has not for many years been cheap (i.e. absolutely) in the southern Californian fruit country, and immigration has been, generally, of the comparatively well-to-do. This fact has greatly affected the character and development of the city. The assessed valuation of property increased more than threefold from 1900 to 1910, being $276,801,517 in the latter year, when the bonded city debt was $17,259,312.50. Since 1896 there has been a strong independent movement in politics, marked by the organization of a League for Better City Government (1896) and a Municipal League (1900), and by the organization of postal primaries to secure the co-operation of electors pledged to independent voting. Since 1904 the public school system has been administered by a non-partisan Board of Education chosen from the city at large, and not by wards as theretofore. Los Angeles, like all other Californian cities, has the privilege of making and amending its own charter, subject to the approval of the state legislature. In 1902 thirteen amendments were adopted, including provisions for the initiative, the referendum and the recall. The last of these provides that 25% of the voters choosing a municipal officer may, by signing a petition for his recall, force a new election during his term of office and thereby remove him if another candidate receives a greater number of votes. This provision, introducing an entirely new principle into the American governmental system, came into effect in January 1903, and was employed in the following year when a previously elected councilman who was "recalled" by petition and was unsuccessful in the 1904 election brought suit to hold his office, and on a mere technicality the Supreme Court of the state declared the recall election invalid. In 1909 there was a recall election at which a mayor was removed and another chosen in his place.