Mining, particularly for gold and silver, is responsible for one of the largest transfers of population from the eastern to the western parts of what became the United States.  In addition to mining, the great migration of settlers from the east to west, in what was then called in the mid-19thC the "Oregon Territory," brought tens of thousands of American families to the western territories.

Until the mid-19thC., the western territories (that is, west of Missouri) were relatively sparsely populated by Europeans, mostly by small groups of hunters who were following the beaver-skin trade, which, at the time, was the most profitable "business" in the western territories.  There were settlers west of Missouri in very small numbers, but hostile relations with the indigenous peoples discouraged large-scale migration into what were then mostly Indian-occupied lands.

In 1849, however, with the discovery of gold at Sutter's Fort just outside what is now Sacramento, California, the true westward migration began.  Over the course of about ten years, more than 100,000 people began the trek to the California gold fields, and following those people were families and, more important, all of the infrastructure necessary to support the both the mining industry and the development of commerce in several states, including Oregon, Washington, California, Nevada, Colorado, and Arizona.  Each of these states, at one time or another, had significant mining activity from the mid to the late 19thC and beyond. 

After the California Gold Rush played itself out in the 1850's, intensive mining activity--and all the commerce and social systems needed to support it--continued to develop especially in Nevada and Colorado, which also saw the beginning of extensive railroad systems to carry people and goods to the western territories on a cost-effective and relatively quick basis from the east.  Many of the west's major cities--San Francisco and Denver, in particular--are the direct result of mining activity in California and Colorado.

Later in the century, after the Civil War, a gold strike in the Dakotas brought another influx of mining activity and tens of thousands of miners, all of whom needed a place to live, created a "mini-boom" in what is still a relatively hostile environment.  Because these miners were encroaching on Indian territory and needed protection, the U. S. Army--represented most famously by General Armstrong Custer (after the Civil War a Lieutenant Colonel)--began its final wars against the American Indians, finally winning those wars by either killing the remaining Indians or moving them onto reservations--out of the way of miners and settlers who were pouring into what used to be Indian lands.  One can argue (successfully) that mining activity in part caused the American Indian culture to collapse, which then allowed "civilization" to replace it in the western territories.