A mineral that forms only under specific pressure and temperature conditions, and that therefore provides information about the metamorphic history of the rock in which it is found. The minerals kyanite, andalusite, and sillimanite are examples of index minerals.
What Bizarre Error Gave California Its Name?When the Spanish first began exploring the Pacific Coast of North America, they mistakenly believed that California was an island. (Some of the earliest and most fascinating maps of the state depict it as separated from the mainland.) This is considered one of the greatest, albeit short-lived, cartographic errors. Early mapmakers began labeling the “island” as California, the name of a mythical island in a book …
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