THE ACFT COLLIDED WITH ROCKS AND SAGEBRUSH DURING A FORCED LANDING AFTER THE ENGINE LOST POWER. THE ACFT ENCOUNTERED INDUCTION AND AIRFRAME ICING DURING CRUISE. THE PILOTS WEATHER BRIEFING FORECAST THE ICING WHICH HE ENCOUNTERED. THE BRIEFING TOOK PLACE ABOUT 12 HOURS BEFORE DEPARTURE. THERE IS NO RECORD OF ANOTHER BRIEFING PRIOR TO DEPARTURE. WHEN THEPILOT EXPERIENCED A REDUCTION IN MANIFOLD PRESSURE DURING CRUISE IN ICING CONDITIONS HE SUSPECTED INDUCTION ICING. THE ALTERNATE AIR DOORS WERE FROZEN. WHEN THE PILOT FREED THEM THERE WAS A SMALL BANGING SOUND IN THE ENGINES. HE RADIOED THE ARTCC AT SALT LAKE CITY THE HE HAD ICING AND WAS 'DROPPING LIKE A ROCK'. AN EMERGENCY LANDING WAS MADE IN A SAGE BRUSH CLEARING. THE PILOT SAID THE ACFT MANUAL SHOULD BE CHANGED. 'IT STATES THAT CARB ICE IS RARE AND IS CORRECTED BY ALTERNATE AIR. IT DOES NOT INDICATE THAT AFTER THE ICE FORMS IT IS TOO LATE TO RESTORE POWER'.