Popular Science Monthly/Volume 41/May 1892/Dendrites
By M. STANISLAS MEUNIER.
THERE is a universal tendency to seek and sometimes to see in the forms of objects around us representations of the human figure or of animals and plants. Many interesting examples have been recorded and pictured in La Nature of rocks and mountains presenting resemblances to animated forms. We are quite ready to discern in the clouds all sorts of personages; and at periods when superstition has been active, apparitions have been described, the whole existence of which consisted of misinterpreted simple resemblances. Stones have usually been considered especially worthy of attention in this category; in tubercles of sandstone and nodules of flint it is easy to find features analogous with the most various objects. A block of sandstone is exhibited in the forest of Fontainebleau on which one willing to see it may recognize a petrified knight on his horse, all of the natural size. A nodule of sandstone was once brought to me in the geological laboratory of the museum, on which the owner saw the portrait of our Lord on the cross. Some persons are specially ingenious in finding resemblances in flints; and Boucher de Perthes admitted into his Atlas of Celtic and Antediluvian Antiquities a whole series of figures of imitative forms of that mineral.
There is no limit to this line of curiosities. All sorts of subjects may be found—calves' heads, which are quite common, and eyes, birds, fishes, detached hands, feet, and ears, and human profiles. A large flint was kept for a long time at Mendon, on which everybody recognized the bust of Louis XIV. To such accidents M. J. B. Robinet, in 1778, devoted a part of his ingenious Considerations on the Efforts of Nature in trying to make Man (Considerations sur les essais de la Nature qui apprend à faire l'homme). As we turn the leaves of this curious work we see described, in distinct paragraphs, anthropocardites, representing the heart of man; encephalites, or brains; cranoïdes, or skulls; otites, or ear-stones; leucophthalmos, or white eyes; chirites, or hands; stones representing a muscle, and even the olfactory nerve, etc.
The drawing of the distinction between fortuitous resemblances and true fossils was protracted and made difficult by the fact that the two forms are often mingled, sometimes associated in the same specimen or originating in beds having the most essential characteristics in common.
Sometimes, for instance, fossils are reduced to the condition of impressions squeezed between two beds of rock or between two laminæ of a schistose stone. Fishes and insects are found in this condition, and plants in prodigious abundance. Accidental cases of color or structure externally resembling these may be found under similar conditions—more or less complicated figures in which it will be often easy to find such resemblances as clouds or the arabesques of a tapestry give us. Fig. 1 represents an example of this kind, from the Saxonia Subterranea of Mylius (Leipsic, 1709); it is the picture of a stone the fracture of which exhibits spots making out the figure of a fowl with her plumage, comb, and the scutels of the tarsi. A class of accidents Of a different Character is especially fruitful of surprises of the kind under consideration. These are the dendrites, which are very frequent in joints of rocks of all ages, and of which Fig. 2 gives a very exact idea. At Romainville and
Fig. 2.—Dendrite, composed of small crystals of ferriferous oxide of manganese—the acerdesis of mineralogists; found in the Assures of a lithographic limestone. (Specimen from the Museum of Natural History; half the natural size.
Argenteuil, near Paris, we may see in the plaster quarries that all the fissures crossing the beds of marl, whatever their color, white, green, or gray, with which the gypsum is cut up, are darkened with dendrites of various dimensions and sometimes very elegant. These dendrites are likewise found in limestones, chalk, building-stones, lithographic stones, and compact marbles; in sandstones, granite, and various other crystalline rocks. They are not always black; some are the color of rust; some are metallic, and consist of common pyrites between sheets of slate, or copper, or native silver, or gold. Finally, besides superficial dendrites, deep ones are known, which are developed across the mass of the stones. The best-known specimens of this kind are those which make appropriate the special designation of arborized agate (Fig. 3).
This name, like that of dendrites, shows that a vegetable origin was at first attributed to these accidents. Sometimes fancy went further; and Fig. 4 represents, from Mylius, whom we have already quoted, the figure of a dendrite in which the author saw a landscape—a plain traversed by a river and bordered by a chain of wooded hills, and pierced with caves. It is easy to discover that dendrites have none of the characteristics of the vegetable ramifications with which we are at first inclined to compare them, and, when we study them under a sufficient magnifying power, the crystalline structure of most of them appears distinctly. This is especially the case with the black dendrites, which are most, abundant, and is shown in the originals of Figs. 2 and 4, which I have particularly studied, and have been able to produce artificially. It is evident that these dendrites, which consist of a hydrated oxide of manganese—the acerdesis of mineralogists—are the result of a precipitating action exercised by calcareous rocks on water containing traces of metallic salts. Hence we might expect to obtain an imitation of them by placing pieces of marble or lithographic stone in a dilute solution of chloride or sulphate of manganese. The hope of success is all the more legitimate because carbonate of lime has already permitted the imitation in this way of several natural minerals, particularly of limonite, an iron mineral, and bauxite, or the mineral of aluminum. But the experiment has not been successful, and, instead of the desired black deposit, we get only chocolate-brown flakes having no resemblance to the substance of the dendrites.
Seeking for the causes of this want of success, I have found, by analysis, that the dendrites said to be of manganese contain oxide of iron, in minute proportions it is true, but in proportions that seem to be sufficient. And the addition of traces of ferric salts to the solution of manganese salt has really determined the deposit on the limestone surface of a perfectly black compound, presenting in many cases the exact form of the dendrites of Nature. I have in the museum specimens that leave no doubt on the subject, the inferiority of which to the models which I sought to copy is most probably due to the inferior slowness of the process of producing them.—Translated for The Popular Science Monthly from La Nature.