UPDATE 5-California wildfire more than doubles on sixth day

* More than 70 dwellings confirmed destroyed

* Over 6,000 homes under evacuation orders

* Fire crews retreat from key mountain summit (Updates with new damage toll, evacuees, news conference)

By Steve Gorman

LOS ANGELES, Aug 31 (Reuters) - A massive wildfire roaring through mountains north of Los Angeles forced some firefighters to retreat on Monday as the toll of engulfed homes rose sharply and flames menaced Mount Wilson, a broadcasting hub and site of a historic observatory.

Fire officials said at least 71 dwellings had been destroyed since the blaze erupted last Wednesday -- 53 clustered in the foothill community of La Crescenta on the northern fringe of suburban Los Angeles and 18 others first reported as lost on Sunday.

As of Monday evening, a total of 6,300 homes throughout the fire zone were under evacuation orders, authorities said.

The so-called Station Fire more than doubled in size as it burned out of control for a sixth day, charring 105,000 acres (42,500 hectares), up from 42,000 acres (17,000 hectares) late on Sunday, and sending up towering palls of smoke that fouled the air for miles (km) around.

Two firefighters died on Sunday when they were overrun by flames in the Angeles National Forest and rugged San Gabriel Mountains. Nearly 2,600 firefighting personnel, some from as far away as Montana and Wyoming, were battling the blaze.

With no forecast for an immediate break in the triple-digit temperatures and very low humidity that has helped energize the fire, officials pushed back their projected date for full containment of the blaze by one week to Sept. 15.

"This is a very angry fire that we're fighting right now," Fire Commander Mike Dietrich told a news conference. "Until we get a change in the weather, I'm not overly optimistic."

BATTLE FOR A MOUNTAINTOP

Fire crews fought to protect the slopes around the 5,700-foot (1,740-metre) peak of Mount Wilson, home to 50 buildings plus a famous array of telescopes and a critical cluster of transmission towers for broadcasters.

After dousing the area in fire retardant and laboring to clear brush away from structures on the site, they fell back early on Monday to avoid flames expected to sweep the summit.

"They've done everything they can do and it's unsafe for them to be there when the fire hits," Los Angeles County Fire Captain Mark Whaling said.

Elsewhere in the forest, 65 firefighters retreated from a wall of flames advancing on their positions, he said.