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The article from the 1911 Encyclopædia Britannica discusses the distribution, types, and formation theories of coral reefs. Coral reefs are primarily found in tropical seas, with the most massive species thriving in shallow waters where temperatures do not fall below 68°F. The text categorizes coral formations into fringing reefs, barrier reefs, and atolls, each with distinct characteristics. Fringing reefs are close to shorelines, while barrier reefs are larger and further from land, with deeper channels separating them from the coast. Atolls are ring-shaped reefs with central lagoons, often formed around subsiding land masses. The article explores various theories on the origin of coral reefs, notably the subsidence theory proposed by Charles Darwin. Darwin suggested that coral reefs form around islands that gradually sink, with coral growth keeping pace with subsidence, eventually forming atolls. This theory was widely accepted due to its simplicity and applicability. However, it faced challenges from other scientists like Louis Agassiz and Karl Semper, who provided evidence of coral formations in non-subsiding areas. John Murray and Alexander Agassiz further questioned Darwin's theory, suggesting that coral reefs could form on submarine elevations without subsidence. Recent research, including deep borings in atolls like Funafuti, supports Darwin's theory in some cases, showing significant limestone deposits indicative of subsidence. The article concludes that coral reefs can form under various conditions, whether in areas of subsidence, elevation, or stability, and that the atoll or barrier reef shape does not necessarily indicate subsidence.