WHEN decorator Joyce Cain and her husband, Jerry, a construction engineer, found a 542-acre ranch on the summit of Spring Mountain overlooking the Napa Valley, it easily became a home site of irresistible appeal. The challenge of vine growing and wine making appealed to the Cains, especially after soil testing offered abundant evidence that the deep shale would be of incomparable value in the production of the red-wine varieties responsible for the great clarets of Bordeaux, Cabernet Sauvignon, Cabernet Franc, Merlot, Malbec and Petit Verdot. Simultaneously with construction of their new mountain home and a winery, 110 acres were planted on dramatic terracing. They employed young Lester Hardy as wine maker, commissioning him to buy choice Napa Valley grapes and launch the Cain Cellars winery. First releases of Napa Valley Chardonnay from Carneros-region grapes were well received.