In one corner of the Early Life hall of Smithsonian's National Museum of Natural History stand two petrified tree trunks. They are 207 million years old, stony relics of an ancient conifer (Araucarioxylon arizonicum) that once flourished in prehistoric Arizona. Smithsonian curators refer to them as the "Sherman Logs," because they were collected in 1879 at the instigation of Gen. William Tecumseh Sherman, the man who led the March from Atlanta to the sea near the end of the Civil War.

Back in the Triassic period they flourished in what is now known as the Black Forest, part of the 93,492-acre Petrified Forest National Park in Arizona. Prehistoric Arizona was a flat stretch of tropical turf in the northwest corner of a supercontinent known to modern geologists as Pangaea.

The trees towered as high as 200 feet and measured more than 2 feet in diameter. Once felled, they "turned to stone" almost instantly (at least in geological terms, meaning anywhere from tens to hundreds of years). Partly because of the fossil evidence they contain, we know what felled them. Arizona could be hot back in the Triassic period, and it had wet and dry seasons on a grand scale--in keeping with the earth dramatics of the time. During the wet season, megamonsoons swept across Pangaea and turned dry-season streamlets into thunderous, roiling rivers.

One rainy season more than 200 million years ago, an enormous volcano added to the confusion by blowing its stack and overloading the already overflowing rivers with ash and debris. The waters jumped their banks and fanned out in a Ganges-like flood, charging across the floodplain, and toppling the big trees, burying them at least 30 to 40 feet deep. Shut off from air, and thus protected from oxidation, bacteria and fungi, the trees did not rot; the wood was instead gradually replaced by rock-hard minerals like the glassy silica so abundant in volcanic ash.

These minerals began to crystallize between the waterlogged wood's cell walls. As the cell structure itself broke down, crystals formed in those spaces as well. In some logs iron oxides and other minerals provide a rainbow of colors (but logs that still contain some organic material, like the Sherman Logs and most others from the Black Forest, tend to be dark).

After about 20 million years or so Pangaea separated into a northern landmass, known as Laurasia, which eventually split into Eurasia and North America, and a southern landmass, known to geologists as Gondwanaland, which evolved into South America, Africa, Australia and Antarctica. Meanwhile, encased in sediment, the Sherman Logs were alternately covered by a shallow tropical sea or salted by huge sand dunes. Sometime during the logs' interment, geologic uplift, erosion and grumbling earthquakes snapped the brittle tree trunks into surprisingly neat sections. In a process that is still going on, the seasonal downpours that once buried the logs began to expose them, by washing away the earth in which they were entombed.

Time passed. The dinosaurs waxed and waned. Ice ages came and went. Primates began to come into their own--and started collecting things. Thirty-five thousand years ago dawn-age naturalists--Neanderthals, in fact--began arranging fossils and lumps of iron pyrite in orderly rows in a cave--perhaps the first museum. In A.D. 40, Caligula, a tough and often bloody-minded Roman emperor, supposedly ordered his troops to collect seashells for posterity--the "spoils of the sea" on the shores of Gaul. And in the autumn of 1878 another commander, the restless General Sherman, spotted "extraordinary specimens" of "petrification" while traversing the new Arizona Territory. From his headquarters in Washington, D.C., Sherman, who was also a regent of the new Smithsonian Institution, wrote to Col. Peter T. Swaine at Fort Wingate, New Mexico, telling him to collect some "interesting samples" of "petrifications for the Smithsonian."

The general told the colonel exactly what he wanted. "I would like to secure at as little cost of money as possible two logs, say from 20 to 25 inches diameter and six or eight feet long, weighing near a ton each. . . ." Sherman envisioned two logs that "would stand on end, in either side of the Museum. . . . Therefore," he noted, "they should match somewhat in size" and should "be encased with wood for transportation" and addressed to the Smithsonian "on the easiest terms possible, for the Smithsonian is poor in money, though rich in the treasures of knowledge."

In the spring of 1879 Swaine dispatched Lieut. J. F. C. Hegewald of the 15th Infantry with a sergeant, 12 men and two heavy mule-pulled wagons to Lithodendron Wash. At "Navajoe Springs," Arizona, Hegewald reported, the little expedition left the road, cutting across country about 20 miles toward the head of the creek. There, Hegewald wrote, the party met Navajos with "thousands of head of sheep," who thought it strange "that the 'Great Father in Washington' should want some of the bones of the 'Great Giant' their forefathers had killed years ago when taking possession of the country." Hegewald also noted the Navajo regarded the surrounding lava beds as "the remains of the blood" that ran from the giant's wounds.

Thousands of petrified specimens lay scattered on the ground. Hegewald's men found sections of many half-exposed stone trunks and reckoned the whole trees must have been "150 to 200 feet in length and from 2 to 4 1/2 feet in diameter." After finding a section of tree he "thought would please," and its "match," a section that "saw the light for the first time in ages," Hegewald had the two fossil logs laboriously loaded onto the wagons.

At least in retrospect, Lieutenant Hegewald's collecting of the stone logs had thus far been a piece of cake. But on the return trip, the heavy-laden troop got delayed in rough country and occasionally had to stop "to make a road."

Though petrified wood is all but impervious to normal weather, the Sherman Logs, it turned out, got collected not a moment too soon. The Petrified Forest had survived the sieges of eons, but it was no match for the entrepreneurial and acquisitive spirit of Homo sapiens. News of the "stone trees" in Arizona Territory soon began to spread. And no wonder, if people saw the rock-candy descriptions like this one by touring German artist Baldwin Möllhausen, published in 1858: "Trees of all sizes lay irregularly scattered about. . . . great heaps of petrifications gleaming with such splendid colours that we could not resist the temptation" to break off a piece.

According to Maj. John Wesley Powell, later to become director of the U.S. Bureau of Ethnology at the Smithsonian, the Piute believed the petrified logs were spent arrow shafts sent by their thunder god, Shinauav. A few Indians had built dwellings from large blocks of petrified wood. Others chipped out tools and weapons--arrowheads and axes. One man's old dead tree, however, can be another man's fortune. In 1885 it was noted that foreigners had shown particular interest in the trees. An enterprising Russian dealer had paid $500 for a petrified log 28 inches in diameter and 30 inches long; he planned to slice it up to make tabletops. Material from the fallen groves was also snapped up for inlay, paneling, floor tiles, mosaics and jewelry.

Heavier inroads on the precious supply of ex-wood were soon to come. Drake & Co. of Sioux Falls, South Dakota, had trunks hauled out of the forest so chunks could be turned into mantels, clock cases, pedestals, paperweights and other articles of agatized wood for Tiffany & Co. and other eager clients.

The Armstrong Abrasive Company of Denver got the idea of grinding up these trunks to make emery. Armstrong moved its Chicago plant to Adamana, Arizona, a stop on the new railroad line. (Adamana got its name from the only man living there, one Adam Hanna, who prospered as a guide for souvenir- hunting parties bound into remote parts of the Petrified Forest.)

By 1899 a U.S. Geological Survey report declared that if something wasn't done about the situation soon, the ancient forest would go to as "virtual extinction as the buffalo." Seven years later Teddy Roosevelt made the area a national monument, but only in 1962, under President John F. Kennedy, did the Petrified Forest finally become a national park.

Ancient trees like those in Arizona tell us a good deal about the now-lost world in which they flourished. Though they are as hard as quartz, they are pockmarked with telltale signs of life. We can see the tunnels where ancient insects bored into them. Geologist Tim Demko and his colleagues have found hundreds of fossilized hives in the trees, suggesting that an insect resembling the modern bee may have been buzzing about 100 million years before flowers came into existence. These finds, in addition to much fossil evidence of other plants and a variety of animals, have led scientists to conjure up the vision of an entire ecosystem.

Today, the research continues. But its subject matter, apparently, is still disappearing. Despite a guard force of seven National Park Service rangers, and fences, warning signs and the threat of a $275 fine, about 12 to 14 tons of the fossil wood disappears from the Petrified Forest every year. The culprits, according to Chief Ranger Pat Quinn, are usually "your everyday honest visitors" seeking souvenirs.

By Adele Conover