During the prosecution of field work in the Hamilton limestones of Michigan, I became aware of the existence of well marked isolated coral mounds or hillocks in the otherwise stratified and almost unfossiliferous limestones. These coral hillocks are particularly well exposed in the vicinity of Alpena, where several quarries have been opened in them. In his survey of the region Rominger spoke of them as “bubble-like” upheavals of the strata. Recent favorable sections in the quarries permitted the study of these hillocks in detail, when their reef character became apparent. A brief account of the reefs was published in the American Geologist, September,* and a more extended one, with a cross-section, in the annual report of the Geological Survey of Michigan 1901.* Since then I have received from Dr Carl Wiman a copy of his paper on Siluric coral reefs in Gotland, published in 1897, in which he describes . . .