In a discussion of Belt formations and fossils in Glacier National Park, Montana, the writers described and illustrated2 thick, compact beds formed by the supposed alga, Collenia. Similar deposits, also of Proterozoic age, have been described by Twenhofel3 and by Moore,4 and figured by Walcott.5 They, and later formations of kindred nature, have been discussed by Cumings,6 who designates them as “biostromes,” in distinction from true lenticular or ridgelike reefs, for which he uses the term “bioherm” of Cumings and Shrock 7—designations adopted in this paper. Though described examples are not numerous, biostromes appear to be common in Proterozoic deposits and are present in those of the Archeozoic 8 as well. With the doubtful exception of the Kona dolomites described by Twenhofel, however, no references were found to bioherms in rocks of Proterozoic age. Indeed, several paleontologists have suggested that organisms did not acquire . . .