Answer:
The increase in population and the way it has spread across the country have caused the American border to disappear.
Step-by-step explanation:
The United States Census of 1890 determined that the resident population of the United States was a total of 62,979,766, an increase of 25.5% over the 50,189,209 people listed in the 1880 census.
This increase in population has provoked the provision of free land to citizens and railways by selling cheap land to European farmers. This caused a rapid settlement of the Great Plains. As a result the frontier virtually ended in 1890.