Professor Hughes in a brief but very valuable paper in the July Namber of this Magazine calls attention to a class of breccias of so much importance to the geologist that I am tempted to make a summary of notes that have been accumulating for some years and present it by way of a supplement. These breccias, produced in situ by mechanical forces, and subsequently modified by chemical, are very liable to mislead the student, andit is on this account (having myself been so misled in former, and often puzzled in recent years), that I call attention again to the subject—of what perhaps sufficient notice is not generally taken in text-books of geology.