for extraction included drift mining and bell pits. As well as drift mines, small scale shaft mining was used. This took the form of a bell pit, the extraction working outward from a central shaft, or a technique called room and pillar in which 'rooms' of coal were extracted with pillars left to support the roofs. Both of these techniques however left considerable amount of usable coal behind. Archeological evidence in China indicates surface mining of coal and household usage after approximately 3490 BC. The earliest reference to the use of coal in metalworking is found in the geological treatise On stones (Lap. 16) by the Greek scientist Theophrastus (c. 371–287 BC): The earliest known use of coal in the Americas was by the Aztecs who used coal for fuel and jet (a type of lignite) for ornaments. In Roman Britain, the Romans were exploiting all major coalfields (save those of North and South Staffordshire) by the late 2nd century AD. While much of its use remained local, a lively trade developed along the North Sea coast supplying coal to Yorkshire and London. This also extended to the continental Rhineland, where bituminous coal was already used for the smelting of iron ore. It was used in hypocausts to heat public baths, the baths in military forts, and the villas of wealthy individuals. Excavation has revealed coal stores at many forts along Hadrian's Wall as well as the remains of a smelting industry at forts such as Longovicium nearby. After the Romans left Britain, in AD 410, there are few records of coal being used in the country until the end of the 12th century. One that does occur is in the Anglo-Saxon Chronicle for the year 852 when a rent including 12 loads of coal is mentioned. In 1183 a smith was given land for his work, and was required to "raise his own coal" Shortly after the granting of the Magna Carta, in 1215, coal began to be traded in areas of Scotland and the north-east England, where the carboniferous strata were exposed on the seashore, and thus became known as "sea coal". This commodity, however, was not suitable for use in the type of domestic hearths then in use and was mainly used by artisans for lime burning, metal working and smelting. As early as 1228, sea coal from the north-east was being taken to London. During the