Popular Science Monthly/Volume 48/January 1896/The Geological Work of the Air
By M. STANISLAS MEUNIER.
SOME specimens recently received at the geological laboratory of the National History Museum in Paris suggest a study of the action of the wind in geology and of the importance, long overlooked, of wind deposits. The specimens are lavas from the volcano Mauna Loa, which the wind, lashing them before hardening, has reduced to fibers of extraordinary fineness. They might be called bunches of oakum. The mechanism of the formation of these—or Pélé's hairs, or bald men's locks, as they are poetically called—is similar to that by which similar formations are produced in furnaces when the blast is directed against the melted slags. It has been proposed to employ the product thus prepared as a textile material; but it is not well adapted to such use, be
Fig. 1.—Pélé's Hairs: Volcanic Lavas spun by the Wind and resembling Tow; from the Volcano Mauna Loa, Sandwich Islands. Natural size.
cause of frequent abrupt changes in the diameter of the vitreous threads. This peculiarity is shown in Fig. 1, where the little black tears mixed in with the brownish fibrous material are really the knots by which the filaments may at any time be interrupted. If we examine these bald men's locks with a microscope, we shall find the evidences of their wind origin more abundant. Fig. 2 represents them magnified fifteen diameters, and Fig. 3 one hundred diameters. We see in the former of these figures that, notwithstanding their coarse appearance, the filaments are anything but homogeneous in all their parts. The axial region of each of them is usually marked by a sort of thread, either continuous or discontinuous, which sometimes suggests a cylindrical cavity, but is more usually in the condition of scattered granulations. The nature of these included bodies can often be determined with precision; and to select one of the clearest cases, they may not infrequently be perceived acting energetically upon polarized light as if they were true crystals, while they show geometrical forms on larger or smaller parts of their contour. On closer examination it is possible to refer them to grains of pyroxene. In some of the filaments the axial inclusions are of a different character; their perceptibly spherical form and other features identify them with the gaseous bubbles frequently observed in rocks. The
matter which fills the cylindrical cavity, often as long as the filaments themselves, is also gaseous; and so likewise are the bubbles that may sometimes be seen by thousands in the vitreous scales, such as are abundantly represented in an angular plate near the edge of Fig. 2, Another point to be noticed is the way the filaments terminate that we are sure have not been broken after consolidation. They are very rarely drawn out to a fine point without bending, but usually they suffer a more or less abrupt curve or are done up into a knot or a loop, of which Fig, 3 indicates some common forms.
If we lightly shake these locks above a sheet of white paper, a fine dust will fall upon it in which the microscope detects among the finest filaments myriads of brownish, translucent, and sometimes transparent, vitreous pellets. Most frequently they are perfectly homogeneous, but they also often contain inclusions similar to those in the filaments. The perfection of their spherical form is not strictly in proportion to their diameter, as might be supposed; but some of the larger ones are very regular, and some of the smaller ones are more or less pyriform. Many of these are traversed by fine perlitic cracks. Together with the pellets are numbers of the vitreous scales which we have just mentioned. Some are perfectly smooth and uniformly transparent; while others contain bubbles and blisters, and have a rough surface. Sometimes innumerable little vitreous pellets are massed upon them, similar to those which form bunches like bunches of grapes on some of the finer hairs.
I do not think that the mixture of pellets with the slag threads spun accidentally near the nozzles of blowers in factories has been insisted upon. These little balls result from a special action of a gaseous medium; and it is important to remark that we can by their presence recognize the wind origin of the deposits that contain them. They have a bearing upon some observations of M. Gaston Tissandier, who has found similar globules in atmospheric sediments in very different localities. These spherules have evidently originated from the action of the air upon the fluid matter developed upon the surface of meteorites during their passage through the atmosphere. They should be formed in considerable numbers at every fall of meteors, and their small volume is favorable to their remaining suspended in the air for a long time, and to their being carried by the wind to considerable distances. This explains the quantities of them found in the ocean bottoms. All marine sediments that have been carefully examined yield globules of this kind; and, as is shown by the common researches of M. Tissandier and myself, they were equally abundant in the ancient geological seas. Thus, to cite an example that has struck us very forcibly, the green sand extracted from the artesian well of Passy, at five hundred and sixty-nine metres below the surface, and which is of the Albian (lower Cretaceous, Potomac group), is full of spherules as perfect as those which are extracted from the dust that has accumulated in the towers of the Cathedral of Paris.
The mechanism of the production of these globules is rendered very evident by their abundance in certain industrial residues, especially in the iron oxide produced by hammering, and in the product of the combustion of iron in oxygen. Evidently the melted oxide spread into laminas by mechanical projection goes through the same capillary action as gives their form to soap-bubbles. We can study all the details of the phenomenon with greater facility if we have recourse to substances much easier to melt than oxide of iron. I have had occasion to do this with the experimental products which M. Daubrée has asked me to examine, and which have been formed in his investigations of the perforation of rocks by gaseous explosions. In these experiments, channels were opened through the granite by the gases of nitroglycerin, showing on their vitrified surfaces all the stages, from the drawing out of a thin pellicle of melted glass to the formation of a perfect spherule. Grains of two different categories may be distinguished with the microscope in the dusts produced in the trituration of rocks by the violent passage of gaseous explosions. Some can not be distinguished from those produced by simple mechanical pulverization. But, besides these materials—angular grains of quartz, feldspar, and mica—we find abundance of perfect, or nearly perfect, spheres, opaque and black or slightly translucent and brownish, with shining surface and often the characteristic little neck. Identical elements are found in the dust derived from different rocks that have been submitted to experiment, but with features that vary in each of them.
Dust obtained from the lava of Vesuvius exhibits the globuli-form character in the highest degree. Nearly all the matter is in the condition of black globules of various dimensions, but always very light, and sometimes having a tubulure. The abundance of these globules is manifestly associated with the relatively easy fusibility of the rock, which is also represented in the constitution of the general glaze with which all the parts that have been in contact with the incandescent gases are covered. The identity of these globules with those that exist so abundantly in the atmospheric dusts and marine sediments can not be contested. Till now, says M. Daubrée, the general opinion, and the only one possible, has been to connect the origin of these globules with the arrival of cosmic masses in the atmosphere; and we may add now to the arguments already presented in support of this thesis, the results afforded by the gaseous trituration of meteoric rocks. The dust furnished by a stone cylinder that fell from the sky in 1888, at Pultusk, was marked by innumerable globules, associated with fragments of peridote and entastite, and with metallic granules which had preserved their ramified form and their adherence to lithoid minerals. What we have said shows also that terrestrial rocks, as well as meteorites, may engender these globules. Hence the arrival of meteorites into the atmosphere incontestably contributes greatly to the production of the brilliant globules so abundant in aërian and aqueous sediments. It seems also to be established that the openings of diatremes have an active part in the matter. The spherules concomitant with the gaseous erosion of granites and other rocks, thrown up into the atmosphere to the great height reached by fine volcanic ejections, may be sustained in the air for a very long time, and fall to the earth at great distances. In support of this opinion it will be recollected that in the basins of seas, the corpuscles of which we are speaking, to which MM. Rénard and Murray do not hesitate to ascribe an extraterrestrial origin, are generally associated with clearly volcanic products.
In order to elucidate completely the origin of the globuliform matters, I have placed melted wax in a pipette with a capillary end, and blown the jet into a vessel of cold water. The product had all the characteristics of the globules of which we are trying to explain the origin, some hollow and having little necks like the meteoric spherules, others full and joined together like the associated spherules in the bald men's locks in the crater of Mauna Loa.
The geological importance of atmospheric sediments is marked in these days in many ways. The facts with which we have been occupied will contribute to illustrate it still more.—Translated for the Popular Science Monthly from La Nature.