It was billed by the Fairfax County Committee of 100 as a debate between the county executive and a noted local consultant over the future of the county's tax system.

Instead, last week's encounter between the county's J. Hamilton Lambert and consultant Philip M. Dearborn, vice president of the Greater Washington Research Center, saw the two quickly square off over a Dearborn study released last year that warned that the county could face a $100 million deficit by 1987.

There was no mistaking how the county executive felt about it. The report was like a "snapshot of the sky," Lambert charged.

"The validity of that picture is of that moment," he said. "Clouds come and go. The sun appears." But, he said, the report failed to take into account budget changes by the county.

As for Dearborn, he insisted the report has been misinterpreted: because the county is required by law to have a balanced budget, it could never actually have a $100 million deficit. He said he was merely attempting to show "the financial pressures" on the county to goad it into making changes.

"Obviously it worked," he said, noting as an example the 3 percent salary cap imposed on county employes' wages.

But County Board Chairman John F. Herrity, a Republican, who was in the audience, said the board approved the salary cap long before Dearborn's report was released. "The government did not take that action in response to the report," he said.

Democratic Annandale supervisor Audrey Moore, who has frequently crossed swords with Herrity, praised Dearborn's study.

She also noted that at the previous board meeting the supervisors had approved more than 30 new staff positions, yet local revenues are not growing and state and federal monies are being cut.

Lambert answered that the county would be reimbursed for many of the jobs by state or other funds. He added that the county would examine all possible methods of increasing its revenues, suggesting hypothetically at one point that the county could even examine whether it would be appropriate to charge for library services.

"I can just see tomorrow's paper," said Lambert when he reflected on what he'd just said.

The invitation to the forum had been accompanied by a summary of a 1981 Montgomery County study that concluded that Fairfax and Prince Williams counties have the most regressive state and local tax structures in the metropolitan region and that Fairfax County had the highest average tax burden for every income group.

Lambert also challenged that report, telling the group "there were two significant flaws in the study. The median housing values used in the study and the median family income used in the study were inaccurate."

The meeting ended on a conciliatory note, however, with Dearborn saying: "I do not have a concern about Fairfax County. Of all the governments in the area, Fairfax County is the most well-positioned to cope with this gap."

"We are dealing with an unknowing economic situation," said Lambert, who noted that the county may very well face some tough decisions.

"By and large the buck will stop with the local governing body," he said.