The Black Country is an area of England's Midlands. It is mainly urban, covering most of the Dudley, Sandwell and Walsall Metropolitan Boroughs. The towns Dudley and Tipton are generally considered to be the centre. The road between Wolverhampton and Birmingham was described as "one continuous town" in 1785. The area was one of the Industrial Revolution's birth places. Either the 30-foot-thick coal seam close to the surface or the mix of coalworks, cokeworks, ironworks, glassworks, brickworks and steelworks (which produced high levels of soot and air pollution in the air at the time) led to the area's name, which was first recorded in the 1840s. The Black Country has no single set of defined boundaries. Some traditionalists define it as "the area where the coal seam comes to the surface – so West Bromwich, Coseley, Oldbury, Blackheath, Cradley Heath, Old Hill, Bilston, Dudley, Tipton, Wednesbury, and parts of Halesowen, Walsall and Smethwick or what used to be known as Warley". There are records from the 18th century of shallow coal mines in Wolverhampton, however. Others have included areas slightly outside the coal field which were associated with heavy industry. Bilston-born Samuel Griffiths, in his 1876 Griffiths Guide to the Iron Trade of Great Britain, stated "The Black Country commences at Wolverhampton, extends a distance of sixteen miles to Stourbridge, eight miles to West Bromwich, penetrating the northern districts through Willenhall to Bentley, The Birchills, Walsall and Darlaston, Wednesbury, Smethwick and Dudley Port, West Bromwich and Hill Top, Brockmoor, Wordsley and Stourbridge. As the atmosphere becomes purer, we get to the higher ground of Brierley Hill, nevertheless here also, as far as the eye can reach, on all sides, tall chimneys vomit forth great clouds of smoke". Today the term commonly refers to the majority of the four metropolitan boroughs of Dudley, Sandwell, Walsall and Wolverhampton although it is said that "no two Black Country men or women will agree on where it starts or ends".