Around 68 % of the working population work in agriculture. Cocoa, coffee, cotton, caoutchouc, oil palm trees, and bananas are cultivated for exporting. Livestock breeding is an important factor in the north. The forests (approx. 42 % of the state area) yield caoutchouc and exotic woods. Natural resources include tin, iron, gold, silver, and titanium; the bauxite reserves in the Adamawa highlands and the natural gas and petroleum found in the coastal regions are of considerable importance, currently constituting almost half of the total exports. Industries are being developed; the aluminum works near Edea and the petrol refinery near Limbe are the principal large-scale industrial operations. Development of what is an already rather fine transport infrastructure is rapidly progressing. The Trans-Cameroon railway up to Ngaoundere was completed in 1974, construction is being continued to reach N'djamena (Chad Republic), primarily being intended to further the economic development of the hinterland. The main port of Douala is connected with the other major cities through railway lines and also has an international airport.