Registered production of silver in the districts of Zacatecas and Sombrerete is shown in Tables 4 and 7. As notes to those tables indicate, production figures are taken from accounts of the royal Treasury offices in the two towns. These accounts, of which the essential purpose was to record the amount of tax paid by the miners on the silver they produced, are the only source of continuous information on output. And since the purpose of this chapter is to account for the observable variations in silver output, it will be as well to see if the Treasury records are a reliable indicator of production. What was the rate of taxation? How were accounts drawn up? And – the fundamental question – how far was the Crown defrauded of its dues on silver?

In Spain and the Empire, subsoil rights belonged to the Crown. But it was obviously impossible for the Crown to organise mining as a state concern, so that in practice all subjects were free to prospect for mines and work them to their own advantage, provided they paid a proportion of their output to the Crown. The proportion varied in time and place; but in New Galicia during the period of this study it was a tenth – el diezmo. The tax was collected by the Treasury officials at the Real Caja.