Rep. Herbert E. Harris (D-Va.) plans to hold early hearings on a bill that would provide enough federal assistance to ensure completion of the entire 100-mile Metro subway system by 1985.

Harris, who once represented Fairfax County on the Metro board, introduced legislation last week that would authorize $1.7 billion in direct appropriations from the U.S. Treasury, plus support for paying off $1 billion in Metro bonds and subsidizing rail operations.

U.S. Secretary of Transportation Brock Adams, in committing the Carter Administration last month to the goal of finishing the 100-mile system, said he would oppose direct appropriations because "we don't think such a bill could carry."

Adams proposed that federal money, other than planned transfers from interstate highway projects, should come from existing programs for aid to mass transit projects nationally. That would put Washington in competition with other cities for federal dollars.

Harris told a Capitol Hill briefing session that there is no chance his bill can be enacted by Congress this year.

But the said the hearing will provide a forum for testimony by federal and regional officials, who agreement on the financing package is vital. Harris is chairman of the House D.C. Regional Affairs Subcommittee, which will hold the hearing, probably later this month.

The original financing program for Metro, which allowed construction to begin in 1969, was based on a direct appropriation by Congress of nearly $1.5 billion, supplemented by Metro's own sale of $1 billion in bonds that - it was then hoped - would be repaid from fare collections.

But as the cost of the 100-mile Metro system soared beyond on early price tag of $3 billion, hope for such repayment evaporated. Continued construction was financed from transfers from abandoned interstate road projects.

Under a revised financing proposal released last month by Metro, the full system would cost $6.7 billion if completed by 1985.

About $3.7 billion in funds already is committed to the project. Of the balance, about $2.4 billion would be in federal assistance. Harris' bill would provide $1.7 billion of that, and the remainder would come from interstate transfers. The area's local governments would have to provide $600 million.

The federal share would be 80 percent and the local share 20 percent of the cost, the same as under the national transit aid program.

Harris' bill would authorize the U.S. government to pay 80 percent of the principal and interest on the $1 billion in Metro bonds, with payments to be made into a special fund starting in 1997.

The measure also would provide a special annual subsidy for Metrorail operations, based upon the value of the system to the operations of the federal government, a Harris aide said. The subsidy would start a $20 million in 1880 and increase $30 million by 1987.

The subsidy is in addition to contributions from the area' local governments and from a national transit aid program.