[Footnote 22:"Bulletin Geol. Soc. Belgium," vol. ix. (1895, p. 3). Also notice in _Geol. Mag._, July, 1895, p. 329.] I regard these observations as of the utmost importance in reference to the relations of dolomite with fossiliferous limestones, and especially with those of the Grenville series. The waters of the Laurentian ocean must have been much richer in salts of magnesium than those of the present seas, and the temperature was probably higher, so that chemical changes now proceeding in limited lagoons might have occurred over much larger areas. If at that time there were, as in later periods, calcareous organisms composed of aragonite, these may have been destroyed by conversion into dolomite, while others more resisting were preserved, just as a modern _Polytrema_ or _Balanus_ might remain, when a coral to which it might be attached would be dolomitized, or might even be removed altogether by sea-water containing carbonic acid. There is reason to believe that this last change sometimes takes place in the deeper parts of the ocean at present. This would account for the persistence of Eozoon and its fragments, when other organisms may have perished, and also for the frequent filling of the canals and tubuli with the magnesian carbonate. The main point here, however, for our present purpose is that, when a calcareous shell or skeleton has been thus infiltrated with a silicate, it becomes imperishable, so that any amount of alteration of the containing limestone short of its absolute fusion would not suffice to destroy an organism once injected with silicious matter. Thus the occasional persistence of silicified fossils in highly metamorphosed limestones is in no respect contradictory to the general fact, that when not preserved by silicious infiltration, they have perished, and this more especially in the case of those whose skeletons are composed of aragonite.