I may mention here a peculiar and interesting structure which has been detected in one of my specimens while these sheets were passing through the press. It is an abnormal thickening of the calcareous wall, extending across several layers, and perforated with large parallel cylindrical canals, filled with dolomite, and running in the direction of the laminæ; the intervening calcite being traversed by a very fine and delicate canal system. It makes a nearer approach to some of the Stromatoporæ mentioned in Chapter VI. than any other Laurentian structure hitherto observed, and may be either an abnormal growth of Eozoon, consequent on some injury, or a parasitic mass of some Stromatoporoid organism overgrown by the laminæ of the fossil. The structure of the dolomite in this specimen indicates that it first lined the canals, and afterward filled them; an appearance which I have also observed recently in the larger canals filled with serpentine (Plate VIII., fig. 5). The cut below is an attempt, only partially successful, to show the Amœba-like appearance, when magnified, of the casts of the chambers of Eozoon, as seen on the decalcified surface of a specimen broken parallel to the laminæ. [Illustration: Fig. 21_a_.] [Illustration: Plate V. _Nature-print of Eozoon, showing laminated, acervuline, and fragmental portions._ This is printed from an electrotype taken from an etched slab of Eozoon, and not touched with a graver except to remedy some accidental flaws in the plate. The diagonal white line marks the course of a calcite vein.] CHAPTER V. THE PRESERVATION OF EOZOON. Perhaps nothing excites more scepticism as to this ancient fossil than the prejudice existing among geologists that no organism can be preserved in rocks so highly metamorphic as those of the Laurentian series. I call this a prejudice, because any one who makes the microscopic structure of rocks and fossils a special study, soon learns that fossils undergo the most remarkable and complete chemical changes without losing their minute structure, and that calcareous rocks if once fossiliferous are hardly ever so much altered as to lose all trace of the organisms which they contained, while it is a most common occurrence to find highly crystalline rocks of this kind abounding in fossils preserved as to their minute structure. Let us, however, look at the precise conditions under which this takes place.