"After going on to Mendoza they were returning by a different pass when they found a petrified forest of fossilised trees, crystallised in a sandstone escarpment showing him that they had been on a Pacific beach when the land sank, burying them in sand which had been compressed into rock, then had gradually been raised with the continent to stand at in the mountains.On returning to Valparaiso with half a mule's load of specimens he wrote to his father that his findings, if accepted, would be crucial to the theory of the formation of the world.