We Easterners have known for some time -- even if we've been a shade reluctant to admit it--that there was more to the opposite side of the country than Tom Mix, the O.K. Corral and the Badlands, despite what we all learned from the movies.

The idea, however, that tutus and toe shoes some day would become as familiar to that distant terrain as 10-gallon hats is of relatively recent provenance. The idea, not the fact--the fact is that classical ballet in this nation grew without respect to geographical boundaries; indeed, the San Francisco Ballet can lay strong claim to being the oldest continuously existing classical troupe in the United States.

The debut last night of Salt Lake City's Ballet West at the Kennedy Center Opera House brought us even further up-to-date on the remarkable burgeoning of the art nationwide over the past two decades.

Though Ballet West wasn't by any means the first western company to appear in the Washington area--those of Houston and San Francisco were among the earlier arrivals--it was the first to make it to the Kennedy Center, and it put on a splendidly impressive show for its initial appearance.

In a program plainly designed to exhibit the company's multiple strengths and directions, the dancers displayed admirable range, both technical and expressive, and a bold, engaging spirit. It's a company obviously worth watching, both now and for the future.

A little historical perspective: Ballet West emerged from the Utah Civic Ballet, founded in 1937 by Willam Christensen, one of the great pioneers of American ballet. In 1968 the much matured troupe was renamed Ballet West; it has since played New York and toured Europe, as well as serving eight neighboring states as a balletic focal point.

When Christensen retired in 1978, the artistic reins were given to Bruce Marks, formerly a dancer of high magnitude, who has overseen a period of skyrocketing growth since. Today Ballet West has a budget in the vicinity of $3 million, and a complement of 39 dancers who give 135 performances yearly--that's big league by any standard.

Where does Ballet West stand in the national picture? It's not, of course, nor would one expect it to be, in the world class of American Ballet Theatre or the New York City Ballet. Rather, it's one of the major regional troupes, along with those in Houston, Boston and Philadelphia (the Pennsylvania Ballet), falling between companies of the second level, like the Joffrey and San Francisco companies and Dance Theatre of Harlem, and those of lesser scope and resources like our own Washington Ballet.

The opening repertory program sampled four signally important branches of company activity--work by promising young choreographers (Helen Douglas' "Mistress of Sorrows"); choreography by artistic director Marks, who's no newcomer to the craft ("Inscape"); a modern classic from the Balanchine treasury ("Allegro Brillante"); and a familiar, brilliant display piece from the Danish repertory ("Etudes").

"Mistress of Sorrows," created last year to Howard Hanson's Sixth Symphony, shows Douglas--now Ballet West's resident choreographer--to have a sure command of the large form, and an imagination teeming with possibility. The choreography, though, for all its workmanship, musicality and ingenuity, falls short of defining an artistic personality.

An abstraction based on the Helen of Troy legend--in this version, Helen is a split personality, the one tragic and remorseful (compellingly portrayed by Leticia Hernandez), the other haughtily erotic (Suzanne Wagner)--the ballet in its body language, its symbolism and its imagistic structure reeks powerfully of Martha Graham, as does the reliance on antique myth. There are some memorably affecting episodes, most notably a long plaintive solo for Hernandez and some swirling ensemble scenes, but on the whole the work succeeds more as pageant than as emotionally involving drama.

Marks' "Inscape" is another abstraction, a duet to a movement from Bartok's Sixth String Quartet that vacillates, like the music, between gnomic pathos and crisp angularity. It's a smartly inventive, compact piece, reflective of Marks' mixed background in modern dance and classicism, and it was danced with notable authority and poignance by Rhonda Lee and Webster Dean.

"Allegro Brillante" was given a curiously laid-back interpretation, nicely responsive to the languorous lyricism inherent in the work's character, but missing out on the Balanchinian rhythmic electricity that is its complementary side. Wagner and Robert Arbogast were the presentable lead couple.

"Etudes," once an American Ballet Theatre warhorse and guest conducted last night by former ABT staffer Akira Endo (now director of the Louisville Orchestra), gave the entire company the chance to show its stuff on stage all at once. It's a lecture-demonstration, minus the lecture, on the rudiments and complexities of classical ballet style and technique.

The staging by Toni and Lise Lander restores a sense of Danish finesse, especially to the adagio passages, danced with wondrous ease, lightness and fluidity by Stacey Swaner. Of the two principal men, Malcolm Burn had more of the requisite line and flash than Bruce Caldwell. On the whole, the troupe came through this exhaustingly virtuosic marathon, despite sporadic faltering and unevenness, most commendably--the standing ovation at evening's end was earned, the hard way.