Complex sentence: He was the first chairman of that Society, and in 1811, when it was more regularly constituted, he was the first president: and in this capacity he served on two subsequent occasions, and did much to promote the advancement of geology.
Split into simpler sentences: He was the first chairman of that Society, and in 1811, when it was more regularly constituted, he was the first president. In this capacity he served on two subsequent occasions, and did much to promote the advancement of geology.