In Jalpaiguri district, the Tarai is known as the Bengal Duars. A large area is inundated, particularly by the Tista which has relatively a large catchment area in that section of the Himalayas which receives heavy rainfall. Near the Himalayas the plain is a rolling one and is higher in elevation. To the south of this relatively high and rolling plain the rivers spread coarse sand over a large area. In the north of Sikkim there is the Great Himalayan Range whose southern slopes are drained by the Tista and its tributary, the Ranjit.
Kanchenjunga (8,598 metres), the third highest peak in the world stands at Sikkim’s western border. In the east, the Donkya Range running in the north-south direction separates Sikkim from Tibet. Being relatively low in height, the water-shed can be easily crossed by the Jelep La (4,385) metres) and the Nathu La. These passes are connected with Gangtok (1,871 metres) the capital of Sikkim and Siliguri in West Bengal Duars via Kalimpong. Three-fourths of Darjeeling district is mountainous and falls within the ambit of the Himalayas.
The district is drained by the Tista, a right-bank tributary of the Brahmaputra. As this mountainous area receives heavy rainfall (305-380 cm. a years) rivers have cut very deep and narrow valleys in this region.
In Duars rainfall is heavy and its annual amount varies from 280 cm.
to 380 cm. Tropical evergreen forests grow due to heavy rainfall. Much of this forest has been cleared. As of now only one- fourth of Jalpaiguri district and the Tarai of Darjeeling district is under forest cover.
Sal grows well in these forests. In Sikkim annual rainfall varies from 380 cm in the south to 76 cm in the north. It rains mainly during the summer monsoon rainy season. During the winter season snow falls on the high mountains.
Varying rainfall and temperature conditions have given rise to a great variety of plants in this region.
Tea is the principal cash crop of the rolling plain adjacent to the Himalayas. As forest soil is rich in humus, clearings are preferred for tea culture.
Pre-monsoon showers are very useful for the increase in the yield considerably. This region has relatively high and easily drained plains, humus-rich soil, adequate rainfall and heat, which favour tea plantations. Of this total area under tea, nearly two-thirds is in Jalpaiguri district and one-third in Darjeeling district. In Sikkim only 15 percent of the total area is under cultivation.
The sown area is restricted to the terraces cut laboriously along the slops. Wet and warm southern Sikkim produces cardamom the leading cash crop of Sikkim. In Darjeeling most of tea estates are locate close to road connecting Siliguri with Lebong via. Darjeeling. This area produces some of the finest varieties of tea in world.
Processing of tea is evidently the principal industry. Extraction of wood, mainly sal from the forests is another industry. Most of the tea factories are located along the railway running between Naksalbari and Alipur Duar for easy transportation. A plant manufacturing single superphosphate is located at Jalpaiguri. There are few minerals in Sikkim. The coal and copper are of some significance.
At present the Sikkim Mining Corporation, Rangpo mines copper ore near Rangpo and produces some copper, lead and zinc.