On April 10, 2000, about 1130 hours Pacific daylight time, a Cessna 305A, N4583V, sustained substantial damage when it collided with a boulder after veering off the runway while landing at Westover Field, Jackson, California. The private pilot/owner operated the airplane under the provisions of 14 CFR Part 91. The pilot and one passenger were not injured. The local personal flight departed Jackson about 1030. Visual meteorological conditions prevailed and no flight plan was filed. The pilot reported the winds were 270 at 4 knots, and runway 19 was in use. He stated he flared high and the airplane bounced, so he added power. As the airplane settled to the runway in a 3-point landing, the tail swung to the left. He tapped the left brake in an attempt to realign the airplane and the airplane suddenly veered left 30 degrees to the runway. The pilot applied right aileron, right rudder, and added power. He departed the runway about 20 degrees off the runway heading and collided with a boulder 30 feet from the runway edge, shearing one main landing gear and buckling the fuselage. He stated he recently installed crosswind gear on the airplane.