A320 ACFT ON VISUAL; NIGHT APCH INTO LAS COMMENCED DSCNT PRIOR TO VOR INBOUND COURSE AND RECEIVED A GPWS FROM TERRAIN THAT WAS LESS THAN 1000 FT BELOW THEM. FLC DID A GAR. RPTR FO A NEW HIRE AND INEXPERIENCED ON ACFT AND UNFAMILIAR WITH ARPT.

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I HAD FLOWN A VERY SIMILAR APCH THE WK BEFORE AND; WHEN I HAD BEEN CLRED FOR A VISUAL APCH I HAD STAYED AT 6000 FT UNTIL ESTABLISHED ON THE FINAL TO ENSURE TERRAIN CLRNC. I WAS NOT TERRIBLY FAMILIAR WITH THE AREA S OF THE MCCARRAN ARPT; SO I THOUGHT IT BETTER TO BE SAFE THAN SORRY. WHAT HAD RESULTED WAS A STRUGGLE TO GET DOWN AND SLOW DOWN ONCE I HAD TURNED FINAL. IN ANTICIPATION OF THIS SAME SCENARIO; I BEGAN TO SLOW TO 160 KTS WHILE ON DOWNWIND LEG. I EXPLAINED WHAT I WAS DOING AND WHY I WAS DOING IT TO THE CAPT. THE AIRPLANE AHEAD OF US WAS GIVEN A BASE TURN AND THEN A TURN ONTO FINAL WITH A CLRNC FOR A VISUAL APCH. AS THE AIRPLANE PASSED ABEAM OUR POS; THE CTLR CONFIRMED THAT WE STILL HAD HIM IN SIGHT AND PROCEEDED TO CLR US FOR THE VISUAL APCH AS WELL. I STARTED MY BASE TURN; AND THE CAPT COMMENTED THAT HE THOUGHT WE COULD SAFELY DSND TO 5000 FT. I AGAIN REITERATED MY DESIRE TO REMAIN AT 6000 FT UNTIL FINAL; BUT HE SEEMED VERY SURE THAT IT WAS SAFE. I ACQUIESCED AND COMMENCED A DSCNT TO 5000 FT. ON OUR NAV DISPLAYS THE ALT 4100 FT WAS DEPICTED AS A CONSTRAINT OVER THE FINAL APCH FIX. WE WERE ON A DOGLEG TO JOIN THE FINAL RIGHT AT THE FINAL APCH FIX AND WE WERE ONLY A COUPLE OF MI FROM IT AS WE LEVELED AT 5000 FT. THE CAPT STATED THAT WE WERE GOOD TO DSND NOW TO 4100 FT. I COMMENTED THAT THE AIRSPACE THAT PROTECTED US AT 4100 FT WAS ONLY VALID ONCE WE WERE ESTABLISHED ON FINAL AND OVER THE FIX INBOUND. HE SAID HE WAS SURE IT WAS SAFE; HE COULD SEE THE ARPT CLEARLY; THE AIRPLANE AHEAD OF US WAS DEFINITELY BELOW OUR ALT AND HE HAD DONE THIS APCH NUMEROUS TIMES. THE CAPT SEEMED VERY CONFIDENT AND NOTHING IN HIS MANNER SIGNALED THAT I SHOULD BE AT ALL CONCERNED ABOUT HIS JUDGEMENT. I REMEMBERED THINKING EARLIER THAT HE SEEMED LIKE A REALLY GREAT GUY TO FLY WITH: VERY PROFESSIONAL AND SELF-ASSURED; WITH VERY GOOD PEOPLE SKILLS TOO. I SELECTED 4100 FT IN THE ALT ALERTER AND STARTED DSNDING. ALMOST IMMEDIATELY THE CAPT SAID SOMETHING ABOUT A MOUNTAIN BEING VISIBLE OUTSIDE THE WINDOW. I LOOKED OUT AND OUR LNDG LIGHTS WERE CLEARLY ILLUMINATING A LARGE PEAK BELOW OUR NOSE. THE CAPT SAID; 'THE RADIO ALT IS SHOWING 1000 FT; LET'S GET OUT OF HERE!' I DISCONNECTED THE AUTOPLT AND INITIATED A CLB AT TOGA THRUST BACK UP TO 6000 FT; SHORTLY AFTER I STARTED THE CLB; THE GPWS CALLED 'TERRAIN; TERRAIN!' WE ADVISED ATC THAT WE HAD INITIATED A GAR DUE TO A GPWS WARNING; AND WE WERE RE-SEQUENCED FOR ANOTHER VISUAL APCH. THIS TIME WE LANDED UNEVENTFULLY. AS THIS WAS THE BEGINNING OF A 4 DAY TRIP FOR US; THE CAPT AND I HAD A LOT OF TIME TO TALK ABOUT OUR MISTAKES. THE CAPT ADMITTED TO ME WITHOUT HESITATION THAT HE SHOULD NEVER HAVE INSISTED THAT I CONTINUE MY DSCNT. ON SEVERAL OCCASIONS I HAD VOICED MY DISCOMFORT WITH THE TERRAIN; BUT HE HAD ALLOWED HIS GREAT EXPERIENCE AND MY RELATIVE LACK OF EXPERIENCE TO AFFECT HIS JUDGEMENT. DURING THE APCH I HAD MY VOR DME RWY 1R CHART CLIPPED TO MY WINDOW SILL; AND WE FELT THAT THERE WAS SOME AMBIGUITY ABOUT TERRAIN CONTOURS DEPICTED ON THAT CHART AS COMPARED TO THE AREA CHART FOR LAS VEGAS. THE APCH CHART SHOWS A CONTOUR INTERVAL MARKED 3000 FT; AND THE SAME INTERVAL IS MARKED 4000 FT ON THE AREA CHART. WHEN HE HAD SAID WE WERE SAFE TO 4100 FT; A QUICK GLANCE AT THE APCH CHART HAD LED ME TO BELIEVE HE WAS RIGHT -- I SAW THE NUMBER 3000 FT AND FORGOT THAT IT WAS A CONTOUR INTERVAL EXTENDING FROM 3000 FT TO 4000 FT IN THE AREA WE WERE IN. THE CAPT TRIED TO TAKE FULL RESPONSIBILITY FOR THE MISTAKES MADE; BUT I TOLD HIM THAT I WOULD NOT ACCEPT THAT. AFTER ALL; I WAS MANIPULATING THE CTLS AND FOLLOWING HIS SUGGESTIONS; EVEN WHEN I KNEW BETTER. I ALLOWED MY POS AS A NEW-HIRE PLT; TRYING TO DO WHAT THE CAPT WANTS; TO IMPAIR MY BASIC INSTINCTS OF GOOD AIRMANSHIP AND COMMON SENSE.