A STUDENT PLT IN A CESSNA 152, N4603L, WAS DEPARTING FROM RWY 25 WHILE CESSNA 172, N733HV, WAS ARRIVING FROM THE NORTH. AFTER TAKING OFF, THE STUDENT MADE A RIGHT CLIMBING TURN TOWARD THE NORTH, THEN BEGAN ANOTHER TURN TO THE EAST IN ACCORDANCE WITH PUBLISHED VFR PROCEDURES. WHILE THE CESSNA 152 WAS IN THE TURN & CLIMBING THRU 800 TO 900 FT MSL, IT & THE CESSNA 172 COLLIDED. RADAR INFO SHOWED THAT THE 172 WAS ON A SOUTHERLY HEADING. THERE WAS NO FULL TIME UNICOM OPERATOR, BUT A PLT WAITING AT THE UNICOM STATION SAID HE HEARD THE CESSNA 172 PLT CALL TURNING DOWNWIND FOR RWY 25. AFTER COLLIDING, THE 152 PLT MADE AN EMERGENCY LANDING ON ROUGH TERRAIN, BUT THE CESSNA 172 WENT OUT OF CONTROL & IMPACTED IN A STEEP DESCENT. ABOUT 8 FT OF THE 172'S RIGHT WING HAD SEPARATED & WAS FOUND JAMMED BENEATH THE 152'S LEFT WING & STRUT. THE COLLISION OCCURRED APRX 2 MI NORTH OF THE ARPT. THERE WAS EVIDENCE THAT THE 152 CONVERGED ON THE 172 FROM THE DIRECTION OF THE SUN. THE 152 PLT DID NOT SEE THE 172 & BELIEVED THE 172 PLT WAS NOT USING A 45 DEG ENTRY.