Though this week's summit meeting between President Bush and Mexico President Carlos Salinas de Gortari was long on symbolism and short on substance, it's potential impact still could be historic.

Yes, more time during the two-day session in Monterrey, Mexico was devoted to social events than to policy discussions. Yes, the Bush-Salinas summit was heavy on effusive expressions of mutual admiration and light on new agreements.But one of the agreements struck in Monterrey at least gives the United States a foot in a door that eventually could lead to our getting more oil from Mexico, reducing U.S. dependence on oil from the volatile Persian Gulf.

Most important of all, of course, is the momentum that this week's summit gave to efforts to produce a free trade pact between the United States and Mexico by 1992.

Negotiations on such a pact won't take place unless Congress authorizes them by next spring. For Congress to refuse to go along would be sheer folly.

Coming on top of the trade agreement the United States signed with Canada two years ago, the proposed new one with Mexico could lead to the creation of a North American common market.

By breaking down trade barriers in this part of the world, the Western Hemisphere would put itself in a better position to compete against an increasingly strong and united European Economic Community.

More immediately, a 1992 trade pact with Mexico would help the United States do a better job of coping in case the General Agreement on Tariff and Trade is unable to liberalize world trade by next month's deadline, a haunting prospect that seems increasingly possible.

Finally, once the United States and Mexico learn how to cooperate on one major front, such an achievement could improve the prospects for dealing more effectively on such matters as drug trafficking and illegal immigration. Clearly, some potentially significant forces were set in motion this week in Monterrey.