The discovery of Potosi revolutionised Upper Peru--as Bolivia was then called. It is probable that the high and inaccessible plateau would have largely escaped Spanish settlement if it had not been for the marvellous riches now offered to Spanish cupidity. Pizarro's original followers came as conquerors and not as settlers. They overran a great and civilised empire whose revenues they proposed to absorb and whose inhabitants they subjected to tribute, but after they had obtained all the gold accumulated in the hands of the Indians there would have been little to have induced them to remain in Bolivia. But as soon as the unprecedented extent of the silver deposit at Potosi was recognised, Bolivia became the greatest source of that metal in the known world and the most important province of the transatlantic dominions of the Castilian king. That one mountain has produced two billion ounces of silver. Even by the early rude processes which the Spaniards found in use among the Indians seventy million ounces were taken out in the first thirty years, and the discovery of quicksilver in Peru, with the invention of the copper-pan amalgamation process in 1575, quadrupled the output. A great mining camp sprang up on the Potosi mountainside; royal officials, contractors, and merchants flocked to this Eldorado; the mountain roads to Lima swarmed with mule trains, carrying down silver and painfully toiling back again laden with supplies; the routes of the Bolivian plateau became the greatest arteries of travel in Spanish America. The year of Gonzalo's execution the city of La Paz was founded in a valley lying in the open plains just south of Lake Titicaca, and soon became a great emporium of Spanish trade. On the fertile plateau to the east of Potosi the city of Charcas flourished and was made the political and ecclesiastical capital of Upper Peru, Potosi being too high for Europeans. Soon other great mines were found, among which those of Oruro, on the south-eastern edge of the Titicaca basin, proved especially rich. Nearly ten thousand abandoned silver mines testify to the activity of the Spaniards in hunting the precious metal, and the total production of silver in Bolivia during the colonial period exceeded three billion ounces. To work these mines the Spaniards ruthlessly impressed the helpless Indians. Each village was required to furnish a certain number of labourers annually. Lots were drawn as if for a proscription, and the unhappy creatures who drew the bad numbers went off to meet a certain death in the dark wet pits and galleries, bidding good-bye to their wives and children like men stepping on the scaffold. The destruction of life was frightful, the official returns made by the officials charged with the impressment demonstrating that in the neighbourhood of Potosi the Indian population fell within a hundred years to a tenth of its original numbers.