Two fetching new dances from the Alvin Ailey troupe

In American modern dance, a time span of 30 years encompasses three generations. Or so it seemed when looking at two new very different productions that recently joined the Alvin Ailey American Dance Theater repertory when they were performed at the City Center here.

''To Jose Clemente Orozco'' was made in 1952 and is entering the Ailey repertory for the first time. ''Fever Swamp'' is hot off the press and will probably become a permanent fixture in the repertory.

The dances will be performed for European audiences this summer, beginning with an engagement in Copenhagen in July, and they will return to the US when the Ailey season begins in September. Made by Lester Horton, a West Coast dance pioneer whose students included Ailey himself, ''To Jose Clemente Orozco'' is deliberately stark and earthbound. Simplicity of gesture and design is the idea, the man's sombrero and woman's serape having equal weight with the steps they perform.

Starkness and simplicity, of course, are new ideas as well as old; these days , Horton's duet could pass for minimalism. What makes ''Orozco'' of its time is that Horton uses these qualities to portray the dignity of mankind.

''To Jose Clemente Orozco'' is meant to be morally uplifting. Taking inspiration from the murals of the famous Mexican painter, Horton transforms Orozco's revolutionaries into Everyman peasants. As they rock under the weight of their clothes and then take flight from an invisible foe, they suggest a suffering that is noble, while the spareness of their movements when they rise from the ground speaks for intensity of purpose.

The idealism inherent in Horton's dance is strange to today's dances. April Berry and Kevin Brown, who danced at the performance I saw, have the right statuesque builds but moved as if they didn't quite believe in the duet's message. One had to read in between the lines to see what the dance was up to. The audience applauded courteously.

Bill T. Jones's ''Fever Swamp'' was received with cries of ''all right.'' This ensemble dance for men goes right to the heart of the contemporary sensibilities, and it's a hit. Jones's dancers aren't peasants; they're street kids - wiseacre, ironic, and out for a good time. They knock the audience for a loop with their energy and sheer abundance of movement.

Most of the choreography is a gust of sign language that, for all its articulation and complexity of gesture, is totally unintelligible. It doesn't matter. The point of this odd, funny hieroglyphics is speed and the emphatic way the men keep pounding it out. Who cares what the men in ''Fever Swamp'' are ''saying'' as long as the energy holds?

And it certainly does, with able help from Peter Gordon's score, called ''Intervallic Expansion.'' Basically a glib affair, the music nevertheless provides an irresistible beat. It keeps ''Fever Swamp'' on a headlong course of bouncy amiability, smoothing over the choreography's repetitions so that you hardly notice them, and always propelling the dancers onward.