Back in Valparaiso, Darwin set out on another trek up the Andes and, on 21 March, reached the continental divide at : even here, he found fossil seashells in the rocks. He felt the glorious view "was like watching a thunderstorm, or hearing in the full Orchestra a Chorus of the Messiah." 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 the 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. After another gruelling expedition in the Andes, while "Beagle" was refitted, he rejoined it and sailed to Lima but found an armed insurrection in progress and had to stay with the ship. Here he was writing up his notes when he realised that Lyell's idea, that coral atolls were on the rims of rising extinct volcanoes, made less sense than the volcanoes gradually sinking so that the coral reefs around the island kept building themselves close to sea level and became an atoll as the volcano disappeared below. This was a theory he would examine when they reached such islands.