The following counties or parts of counties are declared to be devoted chiefly to grazing:

1. (a) That portion of Trinity which is outside of the area described in Section 17125.
2. (b) That portion of Shasta outside of the area which is described in Section 17126.
3. (c) Siskiyou, Lassen, and Modoc.
4. (d) Any county or part of a county which is declared to be devoted chiefly to grazing pursuant to Section 17124.