Some minerals in rocks and organic matter (e.g., wood, bones, and shells) can contain radioactive isotopes. The abundances of parent and daughter isotopes in a sample can be measured and used to determine their age. This method is known as radiometric dating.
The most pressing points of tension with European nations were eliminated during the first half of the century: purchase of the Louisiana Territory from France in 1803 gave the United States control over the heartland of the
