Britain's exports are harder to detect archaeologically, but will have included metals, such as silver and gold and some lead, iron and copper. Other exports probably included agricultural products, oysters and salt, whilst large quantities of coin would have been re-exported back to the continent as well. These products moved as a result of private trade and also through payments and contracts established by the Roman state to support its military forces and officials on the island, as well as through state taxation and extraction of resources. Up until the mid-3rd century, the Roman state's payments appear to have been unbalanced, with far more products sent to Britain, to support its large military force (which had reached 53,000 by the mid-2nd century), than were extracted from the island. It has been argued that Roman Britain's continental trade peaked in the late 1st century AD and thereafter declined as a result of an increasing reliance on local products by the population of Britain, caused by economic development on the island and by the Roman state's desire to save money by shifting away from expensive long-distance imports. Evidence has been outlined that suggests that the principal decline in Roman Britain's continental trade may have occurred in the late 2nd century AD, from 165 AD onwards. This has been linked to the economic impact of contemporary Empire-wide crises: the Antonine Plague and the Marcomannic Wars. From the mid-3rd century onwards, Britain no longer received such a wide range and extensive quantity of foreign imports as it did during the earlier part of the Roman period; vast quantities of coin from continental mints reached the island, whilst there is historical evidence for the export of large amounts of British grain to the continent during the mid-4th century. During the latter part of the Roman period British agricultural products, paid for by both the Roman state and by private consumers, clearly played an important role in supporting the military garrisons and urban centres of the northwestern continental Empire. This came about as a result of the rapid decline in the size of the British garrison from the mid-3rd century onwards (thus freeing up more goods for export), and because of 'Germanic' incursions across the Rhine, which appear to have reduced rural settlement and agricultural output in northern Gaul. Mineral extraction sites such as the Dolaucothi gold mine was probably first worked by the Roman army from c. 75, and at some later stage passed to civilian operators. The mine developed as a series of opencast workings, mainly by the use of hydraulic mining methods.