The National Gallery's American Music Festival ended last night as it began, eight weeks and eight concerts ago, with a concert by the National Gallery Orchestra. The program, this time, was devoted largely to weighty fare, a solemn Suite No. 2 by John Diercks in a neo-Sibelius vein and the sturdy romantic Symphony No. 1 by John Knowles Paine.

Conductor Richard Bales rarely lets a concert go by without some expression of geniality, however, and for this program it was the opening "Country Band March" by Ives that gave the rest a lighthearted send-off. Ives could be deadly serious, but rarely was. This piece is another of his aural impressions of several bands marching around playing different tunes in different tempos and not very expertly at that. All this calculated confusion takes enormous finesse to play correctly, however, and the orchestra, which was in particularly good form last night, did it proud.

Diercks' Suite, written in 1958, is in three ponderous movements of which only the third conveys any sense of rhythmic energy. Lines unfolded slowly, again and again, amid earnest, dense textures. Its modal harmonies held no surprises and the orchestral writing, idiomatic and well crafted, had little variety.

Paine's music is rooted entirely in a European tradition that had not yet glimpsed the influences dormant in the American experience. As a Harvard professor in the last part of the 19th century, Paine stuck to a conservative and idealistic idiom, strongly influenced by Brahms. Last night, the orchestral strings found his lyricism particularly to their liking and projected the long lines with broad transparent beauty.