Forty-two mature marijuana plants growing in plain view on a Chileno Valley ranch were eradicated Friday after a report of a physical altercation drew Marin County sheriff’s deputies to the property, where they inadvertently stumbled on the pot farm. The residents of the Silva dairy, located on Tomales-Petaluma Road at the intersection of Chileno Valley Road, denied any knowledge of the illegal crop, said Lt. Doug Pittman, the Point Reyes station commander, though one family…