49. The importance of the examination and classification of meteorites, with a view to a possible recognition of periodicity of fall of specimens presenting the same characters, need only be mentioned to be appreciated: such a determination is, however, rendered very difficult by the close similarity of structure and composition presented by the great majority of the aerolites of the large chondritic division.
Few aerolites are known which have not been seen to fall.
50. Attention has been already directed to the fact that although many masses of meteoric iron, some of them like that of El Ranchito, near Bacubirito, in Mexico, weighing very many tons, have been found at various parts of the earth's surface, very few of them have been actually observed to fall: in the case of the stony meteorites just the opposite holds good, for they are never very large, and few are known which have not an authenticated date of fall. This may be due to the fact that a meteoric stone is less easily distinguished than is a meteoric iron from ordinary terrestrial bodies, and will thus in most cases remain unnoticed unless its fall has been actually observed; while, further, a quick decomposition and disintegration must set in on exposure to atmospheric influences. The smaller size of the meteoric stones may be due to the greater ease with which they break up on the sudden increase of temperature of their outer surface, consequent on their entry into the earth's atmosphere. The largest meteoric stone preserved in a Museum is one which fell as part of a shower at Knyahinya, Hungary, in 1866: it weighs 647 lbs. and is at Vienna. A larger stone (723 lbs.) fell at Tabory, Russia, in 1887, but was broken to pieces by the impact on the earth; fragments of a still larger single stone, weighing at least 1244 lbs., were found near together at Long Island, Kansas, U.S.A., but the fall was not observed.
The chondrules and their matrix.
51. If we now examine minutely the structure of the meteoric stones, it will be seen that almost all of them appear to be made up chiefly of irregular angular fragments, and that some of them bear a close resemblance to volcanic tuffs. In the large group of chondritic aerolites, chondrules or spherules, some of which can only be seen under the microscope while others reach the size of a walnut, are embedded in a matrix, apparently made up of minute splinters such as might result from the fracture of the chondrules themselves. In fact, until recently, it was thought by some[24] that the chondrules owe their form, not to crystallisation, but to friction, and that the matrix was actually produced by the wearing down of the chondrules through collision with each other either as oscillating components of a comet or during repeated ejection from a volcanic vent of some small celestial body. Chondrules have been observed, however, presenting forms and crystalline surfaces incompatible with such a mode of formation, and others have been described which exhibit features resulting from mutual interference during their growth.
The crystallisation of the chondrules is independent of their form, and must have started, not at the centre, but at various places on their surfaces; Dr. Sorby[25] argued that some at least of the chondrules must once have fallen as drops of fiery rain, and have assumed their shape in an atmosphere heated to nearly their own temperature. The chondritic structure is different from anything which has been observed in terrestrial rocks, and the chondrules are distinct in character from those observed in perlite and obsidian. After much study, Dr. Brezina[26] lends his weighty support to the hypothesis that the structural features of meteorites are the result of a hurried crystallisation: and Prof. Wadsworth[27] accepts the same interpretation.
Some meteoric materials appear to have been altered since their consolidation.
Pane 4o.
52. Since the time of their consolidation some meteoric stones, as Tadjera, appear to have been heated throughout their mass to a high temperature: and in the case of Orvinio, Chantonnay, Juvinas, and Weston, fragments are cemented together with a material having the same composition as the fragments themselves, thus giving rise to a structure resembling that of a volcanic breccia. Others seem to have experienced a chemical change, for some of the chondrules in Knyahinya and in Mezö-Madaras, when examined with the microscope, are found to be surrounded by spherical and concentric aggregations of minute particles of nickel-iron, perhaps due to the reducing action of hydrogen at a high temperature. Others, as Château-Renard, Pultusk and Alessandria, present what in terrestrial rocks would probably be called faults: in some cases the fissures are seen to have been filled with a fused material after the chondrules have been broken and one side of the fissure has glided along the other. These peculiarities of structure suggest that the small body which reaches the earth is only a minute fragment of a much larger mass. It has been suggested that the chondritic structure is of metamorphic origin, and a mere result of enormous pressure on the stony material during the passage through the earth's atmosphere; according to still another view, the structure, though metamorphic, is of extra-terrestrial origin, and due to the quick cooling of a tuff-like stone which has been partially melted, for instance, by the heat from a neighbouring new star or by traversing the hot vapours on the limits of an old one.
Do meteorites reach our atmosphere as clouds of gas or dust?