Article: Once known as "the Gibraltar of the West" and "Fortress Bermuda", Bermuda today is defended by forces of the British government. For the first two centuries of settlement, the most potent armed force operating from Bermuda was its merchant shipping fleet, which turned to privateering at every opportunity. The Bermuda government maintained a local militia. After the American Revolutionary War, Bermuda was established as the Western Atlantic headquarters of the Royal Navy. Once the Royal Navy established a base and dockyard defended by regular soldiers, however, the militias were disbanded following the War of 1812. At the end of the 19th century, the colony raised volunteer units to form a reserve for the military garrison.

Question: How did the colony form a military garrison in the 19th century?
raised volunteer units to form a reserve