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Rh millions. So valuable and successful were these mines that the City of San Luis Potosi, said to have been at one time the second largest centre of population in Mexico, was built near them. However, the exhaustion of the high-grade ores destroyed the prosperity of the city and it was later reduced to the population of a small town. Long before 1890, the high-grade ores had been exhausted and operations were confined to the efforts of Mexican miners scratching around in the old workings for a few remnants of the former great bonanza and in picking over the old dumps and waste material rejected during the bonanza days. Later, an American company built a modern smelter in the city of San Luis Potosi and this enabled the Mexican owners to increase their operations-and handle certain refractory ores to which their own methods could not be applied. Thus a measure of prosperity returned to the camp and was continued until 1903, when it again became necessary to reduce operations to a negligible minimum on account of the low grade of the ores and the primitive methods employed in their extraction. The American company owning the smelter was then induced to take a lease on the mining property at San Pedro under a system of tribute, or royalty, to the native Mexican owners, which is still in effect. Because of large sums expended in development work, new shafts,