 During the California Gold Rush, gold-seekers known as "Forty-Niners" retrieved this gold, at first using simple techniques, and then developing more sophisticated techniques, which spread around the world.Geologic evidence indicates that over a span of at least 400 million years, gold that had been widely dispersed in the Earth's crust became more concentrated by geologic actions into the gold-bearing regions of California.Only gold that is concentrated can be economically recovered.