What is Apex Rule?

Legal Definition
In mining law. The mineral laws of the United States give to the locator of a mining claim on the public domain the whole of every vein the apex of which lies within his surface exterior boundaries, or within perpendicular planes drawn downward indefinitely on the planes of those boundaries; and he may follow a vein which thus apexes within his boundaries, on its dip, although it may so far depart from the perpendicular in its course downward as to extend outside the vertical side-lines of his location 1 but he may not go beyond his end-lines or vertical planes drawn downward therefrom. This is called the apex rule. Rev. St. U. S. § 2322 (U. S. Comp. St 1901, p. 1425); King v. Mining Oo., 9 Mont. 543, 24 Pac. 200.
-- Black's Law Dictionary