 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.