Cal. Gov. Code § 23147
The boundaries of Siskiyou County are as follows:
Commencing on the northern line of the State of California at the northeast corner of Del Norte, being on the summit of a spur of the Siskiyou Range of mountains; thence southerly along the eastern line of Del Norte to the northern line of Humboldt; thence easterly and southerly along the northern and eastern line of Humboldt to the northwest corner of Trinity; thence along the northern boundary of Trinity to the northwest corner of Shasta at Castle Rock; thence due east to the range line between R. 4 and 5 E., M. D. B. & M.; thence north along said range line to the northern boundary of the State of California; thence due west along said state boundary line to the place of beginning.
Ca. Gov. Code § 23147