DURING A CROSS COUNTRY PERSONAL FLIGHT, THE AIRCRAFT EXPERIENCED FUEL EXHAUSTION FOR UNDERTERMINED REASONS AND NOSED OVER DURING THE SUBSEQUENT FORCED LANDING IN A SOFT SAND AND SCRUB BRUSH COVERED AREA. PRIOR TO DEPARTURE OF THE FLIGHT (A MEASURED STRAIGHT LINE DISTANCE OF 75 NM) THE PILOT SAID HE CONFIRMED A FUEL QUANTITY OF 37 GALLONS. ACCORDING TO THE PLT, THE FLT WAS AIRBORNE FOR ABOUT 50 MINUTES WHEN THE 'ENGINE QUIT.' DOCUMENTS PRODUCED BY THE PILOT ESTABLISHED THAT 19 GALLONS OF FUEL WERE PURCHASED AT THE DEPARTURE AIRPORT ON THE DAY OF THE ACCIDENT. EXAMINATION OF THE AIRCRAFT AT THE ACCIDENT SITE BY FAA INSPECTORS REVEALED NO FUEL IN THE TANKS AND ONLY ABOUT A TEASPOON FULL IN THE SYSTEM COMPONENTS FORWARD OF THE FIREWALL. THE FAA INSPECTORS REPORTED NO EVIDENCE OF FUEL LEAKAGE OR STAINS ON THE UNDERLYING AND ADJACENT SOIL OR VEGITATION. NO MECHANICAL MALFUNCTIONS WERE IDENTIFIED DURING THE EXAM OF THE POWERPLANT.