THE REAGAN administration certainly knows how to enliven a summer afternoon. The departure of Alexander Haig as secretary of state and the nomination to replace him of George P. Shultz, a former treasury and labor secretary and budget director, ranks as Ronald Reagan's Mount St. Helens. The political landscape will not be the same again.

The president was prepared to release Secretary Haig, if not to hasten his exit, as graciously as circumstances permitted for an accumulation of reasons. No gavel banger, Mr. Reagan apparently had come to the end of his patience in dealing with an aide who, for all his talents, never adjusted to the style and the tone of this administration. There had been too many bruises, too many reprieves: Bush, Meese, Deaver, Allen, Weinberger, Kirkpatrick, even Clark. The last straw seems to have been the Soviet gas pipeline. For a year, the president felt he had made it clear he wanted to tighten sanctions. Mr. Haig, thinking first of the impact on the alliance, resisted. Two Fridays ago, with Mr. Haig out of town, the president made his decision explicit. Still the secretary thought it should be revised. The sledgehammer fell.

To the end, and characteristically, Secretary Haig saw it his own way. He suggested in his farewell statement that in recent months "foreign policy" (read Ronald Reagan) had shifted from the "careful course" of "consistency, clarity and steadiness of purpose" that he and the president had once laid out. But he had lost more than one very big policy battle, over the pipeline. A proud man, he had also lost, if he had ever really possessed, a role as the chief propounder, or "vicar," of administration foreign policy. People had lost count of the times he had been on the brink of resignation. This time, no one held him back.

We understand the parting, and we regret it. Notwithstanding his role as a square peg in an administration of round holes, Al Haig gave his chief and his country distinguished service. He provided an essential measure of experience, balance and credibility to an administration sorely in need of these qualities to temper the sometime rawness of its ideological drive. On the merits of the pipeline issue, moreover, he made a very powerful case.

Mr. Shultz, the man chosen to succeed him, arrives with the two sets of credentials required of Mr. Reagan's second secretary of state. An old California hand, he long ago earned the confidence of the president in a way that Mr. Haig, a stranger at the time of his appointment, never did; he knows what Mr. Reagan wants in the way of policy and procedure alike. At the same time, he has an international as well as a national stature and thus is well suited to furnishing the assurances of competence that foreigners and Americans are entitled to expect. His broad background in economics would seem to be especially useful, although more attention is likely to be paid, in the confirmation process, to his outspokenly "evenhanded" views on the Middle East.

Mr. Reagan took office mocking his predecessor's management of international affairs. His own has been atrocious. With George Shultz, he has a fresh chance--surely his last--to provide a foreign policy that responds to his direction and that demonstrates to the country that it is doing so. Things are too dangerous, too excited, for Mr. Reagan's detached, chairman-of-the-board approach to be indulged further