Charles Darwin noticed that the geology of the Galapagos Islands suggested that the islands were sinking. Explain what he meant by this statement.

In his journeys on the Beagle during the 1830s, Charles Darwin made some of his most famous observations on the Galapagos Islands. One of his theories developed there was that the islands were sinking, based on the apparent progression of coral reefs surrounding the islands. Darwin hypothesized that as the islands sank (though he did not know why,) a coral reef built up in the surrounding areas. Darwin thought that the different types of coral reefs he saw (known as fringing, barrier, and atoll) represented the stages of this geological change. Though the theory of plate tectonics would not be developing for another century or so, Darwin believed there was some sort of "compensatory action" going on whereby the sinking island contributed to the building-up of coral.