 The lagoons teemed with a rich variety of invertebrates and fish, and he examined the atoll's structure in view of the theory he had developed in Lima, of encircling reefs becoming atolls as an island sank.This idea was supported by the numerous soundings FitzRoy had taken showing a steep slope outside the reef with no living corals below 20–30 fathoms (40–60 m).Arriving at Mauritius on 29 April 1836, Darwin was impressed by the civilised prosperity of the French colony which had come under British rule.