Cal. Sts. & High. Code § 18

“Signature” or “subscription” includes mark when the signer or subscriber can not write, such signer’s or subscriber’s name being written near the mark by a witness who writes his own name near the signer’s or subscriber’s name; but a signature or subscription by mark can be acknowledged or can serve as a signature or subscription to a sworn statement only when two witnesses so sign their own names thereto.