Within the years 1837 and 1839 Darwin read his first paper on the coral islands of the Pacific, and Murchison published the Silurian System, in which he described, under the name of “ballstones,” the reefs of the Wenlock. In 1847 Murchison described the Silurian reefs of Gotland, and did not hesitate to call them reefs. Since then the literature of ancient coral reefs has grown enormously in Europe, but in America the ground which Chamberlin so carefully prepared in his beautiful description of the Niagaran reefs of Wisconsin (1877) has lain too long fallow. There are many brief references to ancient reefs in American literature ; but, outside of the writings of Chamberlin, Grabau, Vaughan and Twenhofel, little appreciation of the significant relations of these bodies or even of their real nature has been evinced. The present writers plead guilty, along with most of their contemporaries, to this lack . . .