in Europe or Asia is several thousands of kilometers. The largest population clusters of North America (the Boston – Washington, San Francisco - San Diego, Chicago – Pittsburgh, and Quebec City – Windsor Corridors) are all thousands of kilometers away from Alberta. Partly for this reason, Alberta has never developed a large presence in the industries that have traditionally started industrialization in other places (notably the original Industrial Revolution in Great Britain) but which require large labour forces, and large internal markets or easy transportation to export markets, namely textiles, metallurgy, or transportation-related manufacturing (automotives, ships, or train cars). Agriculture has been a key industry since the 1870s. The climate is dry, temperate, and continental, with extreme variations between seasons. Productive soils are found in most of the southern half of the province (excluding the mountains), and in certain parts of the north. Agriculture on a large scale is practiced further north in Alberta than anywhere else in North America, extending into the Peace River country above the 55th parallel north. Generally, however, northern Alberta (and areas along the Alberta Rockies) is forested land and logging is more important than agriculture there. Agriculture is divided into primarily field crops in the east, livestock in the west, and a mixture in between and in the parkland belt in the near north. Conventional oil and gas fields are found throughout the province on an axis running from the northwest to the southeast. Oil sands are found in the northeast, especially around Fort McMurray (the Athabasca Oil Sands). Because of its (relatively) economically isolated location, Alberta relies heavily on transportation links with the rest of the world. Alberta's historical development has been largely influenced by the development of new transportation infrastructure, (see "trends" below). Alberta is now served by two major transcontinental railways (CN and CP), by three major highway connections to the Pacific (the Trans-Canada via Kicking Horse Pass, the Yellowhead via Yellowhead Pass and the Crowsnest via Crowsnest Pass), and one to the United States (Interstate 15), as well as two international airports (Calgary and Edmonton). Also, Alberta is connected to the TransCanada pipeline system (natural gas) to Eastern Canada, the Northern Border Pipeline (gas), Alliance Pipeline (gas) and Enbridge Pipeline System (oil) to the Eastern United States, the Gas Transmission Northwest and Northwest Pipeline (gas) to the Western United States, and the McNeill HVDC Back-to-back station (electric power) to Saskatchewan.