Next, they appear to admit that if specimens occur wholly composed of carbonate of lime, their theory will fall to the ground. Now such specimens do exist. They treat the Tudor specimen with scepticism as probably “strings of segregated calcite.” Since the account of that specimen was published, additional fragments have been collected, so that new slices have been prepared. I have examined these with care, and am prepared to affirm that the chambers in these specimens are filled with a dark-coloured limestone not more crystalline than is usual in the Silurian rocks, and that the chamber walls are composed of carbonate of lime, with the canals filled with the same material, except where the limestone filling the chambers has penetrated into parts of the larger ones. I should add that the stratigraphical researches of Mr. Vennor, of the Canadian Survey, have rendered it probable that the beds containing these fossils, though unconformably underlying the Lower Silurian, overlie the Lower Laurentian of the locality, and are, therefore, probably Upper Laurentian, or perhaps Huronian, so that the Tudor specimens may approach in age to Gümbel’s Eozoon Bavaricum.[AR]
[AR] I may now refer in addition to the canals filled with calcite and dolomite, detected by Dr. Carpenter and myself in specimens from Petite Nation, and mentioned in a previous chapter. See also [Plate VIII].
Further, the authors of the paper have no right to object to our regarding the laminated specimen as “typical” Eozoon. If the question were as to typical ophite the case would be different; but the question actually is as to certain well-defined forms which we regard as fossils, and allege to have organic structure on the small scale, as well as lamination on the large scale. We profess to account for the acervuline forms by the irregular growth at the surface of the organisms, and by the breaking of them into fragments confusedly intermingled in great thicknesses of limestone, just as fragments of corals occur in Palæozoic limestones; but we are under no obligation to accept irregular or disintegrated specimens as typical; and when objectors reason from these fragments, we have a right to point to the more perfect examples. It would be easy to explain the loose cells of Tetradium which characterize the bird’s-eye limestone of the Lower Silurian of America, as crystalline structures; but a comparison with the unbroken masses of the same coral, shows their true nature. I have for some time made the minute structure of Palæozoic limestones a special study, and have described some of them from the Silurian formations of Canada.[AS] I possess now many additional examples, showing fragments of various kinds of fossils preserved in these limestones, and recognisable only by the infiltration of their pores with different silicious minerals. It can also be shown that in many cases the crystallization of the carbonate of lime, both of the fossils themselves and of their matrix, has not interfered with the perfection of the most minute of these structures.
[AS] In the Canadian Naturalist.
The fact that the chambers are usually filled with silicates is strangely regarded by the authors as an argument against the organic nature of Eozoon. One would think that the extreme frequency of silicious fillings of the cavities of fossils, and even of silicious replacement of their tissues, should have prevented the use of such an argument, without taking into account the opposite conclusions to be drawn from the various kinds of silicates found in the specimens, and from the modern filling of Foraminifera by hydrous silicates, as shown by Ehrenberg, Mantell, Carpenter, Bailey, and Pourtales.[AT] Further, I have elsewhere shown that the loganite is proved by its texture to have been a fragmental substance, or at least filled with loose debris; that the Tudor specimens have the cavities filled with a sedimentary limestone, and that several fragmental specimens from Madoc are actually wholly calcareous. It is to be observed, however, that the wholly calcareous specimens present great difficulties to an observer; and I have no doubt that they are usually overlooked by collectors in consequence of their not being developed by weathering, or showing any obvious structure in fresh fractures.
[AT] Quarterly Journal Geol. Society, 1864.
3. With regard to the canal system, the authors persist in confusing the casts of it which occur in serpentine with “metaxite” concretions, and in likening them to dendritic crystallizations of silver, etc., and coralloidal forms of carbonate of lime. In answer to this, I think it quite sufficient to say that I fail to perceive the resemblance as other than very imperfectly imitative. I may add, that the case is one of the occurrence of a canal structure in forms which on other grounds appear to be organic, while the concretionary forms referred to are produced under diverse conditions, none of them similar to those of which evidence appears in the specimens of Eozoon. With the singular theory of pseudomorphism, by means of which the authors now supplement their previous objections, I leave Dr. Hunt to deal.
4. With respect to the proper wall and its minute tubulation, the essential error of the authors consists in confounding it with fibrous and acicular crystals, and in maintaining that because the tubuli are sometimes apparently confused and confluent they must be inorganic. With regard to the first of these positions, I may repeat what I have stated in former papers—that the true cell-wall presents minute cylindrical processes traversing carbonate of lime, and usually nearly parallel to each other, and often slightly bulbose at the extremity. Fibrous serpentine, on the other hand, appears as angular crystals, closely packed together, while the numerous spicular crystals of silicious minerals which often appear in metamorphic limestones, and may be developed by decalcification, appear as sharp angular needles usually radiating from centres or irregularly disposed. Their own plate (Ophite from Skye, King and Rowney’s Paper, Proc. R. I. A., vol. x.), is an eminent example of this; and whatever the nature of the crystals represented, they have no appearance of being true tubuli of Eozoon. I have very often shown microscopists and geologists the cell-wall along with veins of chrysotile and coatings of acicular crystals occurring in the same or similar limestones, and they have never failed at once to recognise the difference, especially under high powers.