He set off again and on 1 October, while searching the cliffs of the Carcarañá River, found "an enormous gnawing tooth", and then, in a cliff of the Paraná River, saw "two great groups of immense bones" which were too soft to collect but a tooth fragment identified them as mastodons. Illness delayed him at Santa Fe, and after seeing the fossilised casing of a huge armadillo embedded in rock, he was puzzled to find a horse tooth in the same rock layer since horses had been introduced to the continent with European migration. They took a riverboat down the Paraná River to Buenos Aires but became entangled in a revolution as rebels allied to Rosas blockaded the city. The passport helped, and with Covington, he managed to escape in a boatload of refugees. They rejoined "Beagle" at Montevideo.