California as an Island
With orders from Cortes, Francisco de Ulloa explored both sides of the Baja California peninsula in 1539. It was he who correctly determined that it was not an island. Accordingly, maps of the regions depicted California as part of the mainland for the remainder of the 16th century.
In the early 17th century, Father Antonio de Ascension wrote about a 1602 voyage by another Spanish explorer, which revealed that California was, in fact, an island. Ascensionís account was published as part of a larger work and, within a few years, his errant theory that California was an island began to take hold.
In 1622, a Dutch atlas was published depicting California separate from the North American mainland. By the 1640s, most of Europeís major cartographers were publishing maps showing California as an island. This persisted for over 100 years.
Over time, knowledge of the area increased and solid evidence to the contrary was relayed back to Europe. By the 18th century, some maps began showing California as a peninsula. It was not until 1747, however, when Spainís King Ferdinand issued a decree proclaiming that California was not an island that this issue was officially settled.