There is no question but that limestone may be formed in a number of ways, nor are we yet in a position to determine the relative importance of numerous agents in the formation of the bulk of our pure and impure fine-grained, nonfossiliferous limestones from the pre-Cambrian to the Tertiary. There is plenty of field evidence as to the clastic origin of limestone, and R. C. Wells, Williams, Johnson, and others have shown the possibility of a purely physical precipitation of fine-grained calcium carbonate. Recently Parker D. Trask 2 has stated that he has found “salinity to be a major factor governing the deposition of calcium carbonate.” This statement should be given serious consideration, as it is founded upon 3,000 samples of shallow- and deep-water marine muds collected from all parts of the world. But it should be noted that Trask found that the bottom sediments . . .