Cal. Evid. Code § 1604

A certificate of purchase, or of location, of any lands in this state, issued or made in pursuance of any law of the United States or of this state, is prima facie evidence that the holder or assignee of such certificate is the owner of the land described therein; but this evidence may be overcome by proof that, at the time of the location, or time of filing a preemption claim on which the certificate may have been issued, the land was in the adverse possession of the adverse party, or those under whom he claims, or that the adverse party is holding the land for mining purposes. (Enacted by Stats. 1965, Ch. 299.)