ARES VALLIS, Mars: Sojourner continues its slow progress towards a rendezvous with a rock known as "Yogi" after 10 hours spent aiming its alpha proton X-ray spectrometer at "Barnacle Bill." Preliminary date shows the rock might have been formed more than a billion years ago by a volcano, since it appears to be made of a kind of andesite, the second most common type of lava on Earth. Scientists have been poring over geological information from the rover, looking for clues to how the planet was formed, what changes occurred over billions of years and whether life, however primitive, ever existed there. News that the Ares Vallis plain where Pathfinder landed had once been inundated by a massive flood increased the likelihood that some sort of life existed in the past. Although geologists have known since the Viking landing 21 years ago that floods once swept the planet, Pathfinder has provided the clearest details yet. The water would have covered a swath hundreds of miles wide with hundreds or thousands of feet of churning water, reshaping the planet's surface. "My hope is within the next couple of days to quantify the magnitude of this flood," Pathfinder scientist Michael Malin said.