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A FORMIDABLE ATTACK of "geological indigestion" probably triggered the devastating eruption of Mount Pinatubo in the Philippines last June, according to scientists who were monitoring instruments on the mountain before, during and after the event.

The finding, by a team from the U.S. Geological Survey in Denver led by John Pallister, appears to confirm earlier speculation that such explosive disgorgements can be induced when Mother Nature dumps a sudden dose of super-hot basalt - a thin, very fluid molten rock, or magma - into the belly of a geological formation.In Pinatubo's case, the "belly" - a deep chamber perhaps 3 miles beneath the summit - is normally filled with a reservoir of volatile magma that is thicker and cooler than the intruding basalt. It had been sitting there apparently since the last eruption around 600 years ago, according to the report in the journal Nature.

The basalt destabilized the reservoir and produced a thermal upwelling of magma that scientists detected as prefatory "burps and belches" last April and May. On June 7, it erupted in a dome of lava. Dome fragments show the two types of magma incompletely mixed, USGS geologist Chris Newhall said. The undiluted main reservoir began to blow in a series of explosions from June 12 to the big one on June 15. - Kathy Sawyer (Washington Post)- SCIENTISTS DIGGING at a highway construction site south of Smyrna, Del., have turned up the 17 million-year-old remains of a hornless rhinoceros and two species of early horses the size of an Irish setter. They are calling it one of the richest deposits of land-mammal fossils on the East Coast north of Florida.

"It's a once-in-a-lifetime opportunity for us," said Kelvin W. Ramsey, a scientist with the Delaware Geological Survey. "It's the best fossil site north of Florida for mammals. . . . I think for any period."

Other mammal remains found in the 50-foot-deep excavation have included foot bones and a claw from a chalicothere (pronounced calico-theer), a horselike creature with long front legs, short rear legs and a sloping back. Instead of hoofs, the animal had three-toed feet with large claws.

David J. Bohaska, a paleobiologist at the Smithsonian Institution in Washington, said paleontologists and construction workers also have found fossils identified by the Smithsonian as the remains of early beaver, rabbit, a small peccary and an extinct deerlike animal. - Frank D. Roylance (The Baltimore Sun)