LIKE THE NATION'S Founding Fathers, the progenitors of the conservation movement have included a notable number of gifted writers. John Muir, Bob Marshall, Aldo Leopold and David Brower are worth reading as much for their style as for the substance of their polemics. Of these, the place of honor must go to Muir -- and not only because he came first. (He founded the Sierra Club, which developed into the first and strongest national conservation group, in 1892.) It was he who discovered that even people who had never set foot in wilderness could be rallied to defend it by excellent writing. As he intuited, environmentalists are at heart hopeless romantics, the kind of people who can be mesmerized by the mere description of a mountain or glacier. Banking on this insight, Muir became a lobbyist by pen.

Born in Scotland in 1838, Muir came to Wisconsin with his family in 1849. He attended the University of Wisconsin, taking Latin, Greek and the sciences, but in too unorthodox a fashion to earn a degree. He was an inventor of thermometers and barometers, but his keenest interests were in botany and geology. After years of wandering and working at odd jobs, mostly in the Sierra Nevada, he married at the age of 41 and in between wilderness treks raised fruit near San Francisco. His magazine articles and other efforts were in large part responsible for the establishment of Yosemite National Park. He wrote nearly all of his books during the last decade of his life (he died in 1941); however, many of them, including My First Summer in the Sierra and Travels in Alaska, were based on field notes taken 20 or 30 years before. In reissuing these two books, Houghton Mifflin has apparently reproduced the original plates. The result is a pleasant antique look for the type and photographs.

In photographs Muir has something of John Brown's searing stare, and Muir was inclined to act out the kind of cracked inspiration that most of us suppress. Once in a thunderstorm he shinnied a hundred feet up a Douglas fir to taste the swaying frenzy. Frequently in his travels he set off climbing a peak or tramping on a glacier with no regard for the coming of nightfall. In a nerve-wracking chapter from Travels in Alaska, he recounts how he and a dog named Stickeen ventured out on a vast glacier, "a prairie of ice." By the time they had finished exploring it, they were 15 miles from camp with three hours left until dark. Uncharacteristically, Muir was worried. "After dark, on such ground," he writes, "to keep from freezing, I could only jump up and down until morning." They came to a series of crevasses, which could be crossed only via connecting ligaments of ice -- "bridges" Muir calls them. "In treading the mazes of this crevassed section, I had frequently to cross bridges that were only knife-edges for twenty or thirty feet, cutting off the sharp tops and leaving them flat so that little Stickeen could follow me." After a seeming eternity of harrowing maneuvers, Muir and the dog managed to get down off the glacier and onto solid moraine just at dusk.

When he cares to, Muir can write very well. He has a talent for phrase-making, as when he calls sheep browsing in the Sierra "hoofed locusts." And he often invests his nature descriptions with elan. Along the coast of Alaska, he writes, "you may notice bare rocks just above the water, mere dots punctuating grand, outswelling sentences of islands." Doing a stock wilderness set-piece, the onset of a mountain storm, he parlays his attentiveness into a noble depiction of thunder. "At last the clear ringing strokes are succeeded by deep low tones that grow gradually fainter as they roll afar into the recesses of the echoing mountains, where they seem to be welcomed home."

Muir's pervasive fault as a writer -- and it is not a minor one -- is gushing. Despite his ability to make first-rate metaphors, all too often he falls back on conclusory emotiveness. He pins effusive words like "charming," "superb," "glorious," "majestic," "inspiring" and "celestial" to landscape upon landscape. Perhaps cheap rhapsody was in vogue and suited Muir's evangelical purposes, but it is painful to see him getting by with pedestrian prose when he can produce such evocative descriptions as this: "July 31 . . . the air as delicious to the lungs as nectar to the tongue; indeed the body seems one palate, and tingles equally throughout." Travels in Alaska is much freer of Muir's ejaculations than the Sierra book. He may have realized that Alaska is too grandiose for hosannahs, that only judicious, concrete writing could deliver a sense of its monstrous beauty.