What was the leading economic activity in the West's mountain regions?

Mining was the leading economic activity in the West's mountain regions, especially during the 19th century. This period saw significant geological exploration and the discovery of valuable minerals like gold, silver, and copper, particularly during events like the Gold Rush. As miners flocked to these areas in pursuit of wealth, towns rapidly grew around these mining sites, creating economic hubs focused largely on extracting resources. The mining industry not only attracted settlers to the mountainous areas but also led to the development of infrastructure like railroads to transport these minerals, further solidifying mining's dominance as the primary economic driver in the region.

While agriculture, ranching, and forestry were important in the broader context of the western economy, they did not match the immediate and intense boom that mining brought to these mountainous regions. Therefore, mining is correctly identified as the leading economic activity in these areas during that time.