holder of a debt could use it as a means of payment by transferring it to another party, without cash changing hands. Although it has sometimes been thought that ancient Rome lacked "paper" or documentary transactions, the system of banks throughout the Empire also permitted the exchange of very large sums without the physical transfer of coins, in part because of the risks of moving large amounts of cash, particularly by sea. Only one serious credit shortage is known to have occurred in the early Empire, a credit crisis in 33 AD that put a number of senators at risk; the central government rescued the market through a loan of 100 million HS made by the emperor Tiberius to the banks (mensae). Generally, available capital exceeded the amount needed by borrowers.< The central government itself did not borrow money, and without public debt had to fund deficits from cash reserves. Emperors of the Antonine and Severan dynasties overall debased the currency, particularly the denarius, under the pressures of meeting military payrolls. Sudden inflation during the reign of Commodus damaged the credit market. In the mid-200s, the supply of specie contracted sharply. Conditions during the Crisis of the Third Century—such as reductions in long-distance trade, disruption of mining operations, and the physical transfer of gold coinage outside the empire by invading enemies—greatly diminished the money supply and the banking sector by the year 300. Although Roman coinage had long been fiat money or fiduciary currency, general economic anxieties came to a head under Aurelian, and bankers lost confidence in coins legitimately issued by the central government. Despite Diocletian's introduction of the gold solidus and monetary reforms, the credit market of the Empire never recovered its former robustness. Mining and metallurgy The main mining regions of the Empire were the Iberian Peninsula (gold, silver, copper, tin, lead); Gaul (gold, silver, iron); Britain (mainly iron, lead, tin), the Danubian provinces (gold, iron); Macedonia and Thrace (gold, silver); and Asia Minor (gold, silver, iron, tin). Intensive large-scale mining—of alluvial deposits, and by means of open-cast mining and underground mining—took place from the reign of Augustus up to the early 3rd century AD, when the instability of the Empire disrupted production. The gold mines of Dacia, for instance, were no longer available for Roman exploitation after the province was surrendered in 271. Mining seems to have resumed to some extent during the 4th century. Hydraulic