An interesting example of this striking similarity between a purely mineral deposit and the external form of a plant is afforded by some specimens originally described as impressions of the oldest known fern. The frontispiece to a well-known work on fossil plants, _Le monde des plantes avant l’apparition de l’homme_[152], represents a fern-like fossil on the surface of a piece of Silurian slate. The supposed plant was named _Eopteris Morierei_ Sap., and it is occasionally referred to as the oldest land plant in books of comparatively recent date. In the Museum of the School of Mines, Berlin, there are some specimens of Angers slate on some of which the cleavage face shows a shallow longitudinal groove bearing on either side somewhat irregularly oblong and oval appendages of which the surface is traversed by fine vein-like markings. A careful examination of the slate reveals the fact that these apparent fern pinnules are merely films of iron pyrites deposited from a solution which was introduced along the rachis-like channel. Many of the extraordinary structures described as plants by Reinsch[153] in his Memoir on the minute structure of coal have been shown to be of purely mineral origin. The innumerable casts of animal-burrows and trails as well as the casts of egg-cases and various other bodies, which have been described as fossil algae, must be included among the most fruitful sources of error. It requires but a short experience of microscopical investigation of fossil plant structures to discover numerous pitfalls in the appearance presented by sections of calcareous and siliceous nodules. The juxtaposition of tissues apparently parts of the same plant, and the penetration by growing roots of partially decayed plant débris, serve to mislead an unpractised observer. In sections of the English ‘calcareous nodules’ one very frequently finds the tissue of Stigmarian appendages occupying every conceivable position, and preserved in places admirably calculated to lead to false interpretations. The more minute investigation of tissues is often rendered difficult by deceptive appearances simulating original structures, but which are in reality the result of mineralisation. It is no easy matter in some cases to discover whether a particular cell in a fossil tissue was originally thick-walled, or whether its sclerous appearance is due to the deposition of mineral matter on the inside of the thin cell-membrane. Examples of such sources of error as have been briefly referred to, and others, will be found in various parts of the descriptive portions of this book. [Illustration: FIG. 24. _A._ Section of partially disorganised tissue attacked by some boring animal. _c_, _c_, coprolites; _d_, a tunnel made by the borer through the plant tissue.