Aug. 3, 1888.] 



SCIENTIFIC NEWS. 



117 



garded as a residue of crystallisation whence numerous 

 individual crystals have extracted their constituents In 

 the volcanic glass, black, brilliant, and opaque, and 

 apparently no longer of a crystalline nature, the micro- 

 scope discovers a world of mineral forms. It shows us 

 the various periods of their growth, and the cessation of 

 their development caused by the more or less rapid 

 consolidation of the mass. These strangely-formed 

 rudimentary crystals, the first steps of amorphous matter 

 towards a crystalline state, are formed chiefly in those 

 rocks which have retained most of their glassy nature, 

 and which are consequently homogeneous to the naked 

 eye, e.g., obsidian. 



Owing to the rapidity with which the glassy paste has 

 solidified, the crystals have been unable to fully develop, 

 and their formation has been brusquely arrested. Hence 

 the embryonic crystals which abound in natural glass, and 

 which receive the name of crystallites. Analogous 

 crystals are present in the slag of blast furnaces, bearing 

 close resemblance to those of lava matter. The micro- 

 scope proves this resemblance, for, on close examination 

 of fine sections of slag, rudimentary crystalline forms 

 are seen closely akin to the crystallites of volcanic glass. 



But, as a rule, the crystals do not remain in this 

 embryonic stage. If the lava does not petrify too 

 abruptly, the molecules preserve their freedom of move- 

 ment, even in a semi-liquid mass, and individual crystals 

 of minute dimensions arise called microliths. These 

 microscopical crystals are formed in the heart of the 

 vitreous mass during its gradual solidification. In spite 

 of their infinite minuteness, these little polyhedrons 

 exhibit with marvellous exactitude all the characteristics 

 of their kind, often only fully recognised in far larger 

 specimens of the mineral world, and whose presence in 

 lava would certainly never have been suspected. These 

 microliths in the lava sometimes form a wonderful crystal- 

 line network, which gives a special structure to the rock 

 where they are developed, known as the microlithic 

 structure. 



The dimensions, invariably microscopical, of these 

 microliths, and their disposal in the rocks, clearly prove 

 that they belong to a troubled period, that they were 

 formed at a time when the lava, still in motion, com- 

 menced to solidify — originated, indeed, at the actual 

 moment of eruption. Besides these microscopical 

 crystals and these groups of crystallites, which belong to 

 the last stage of consolidation, the lava contains a certain 

 amount of larger and more developed crystals, which 

 can often be detected by the naked eye. These have 

 been formed under calmer conditions, analogous to those 

 presented by a tranquil fluid where crystallisation could 

 take place but slowly. They were formed in the chemical 

 molten bath when it was still hidden in its subterranean 

 reservoirs. This gradual growth is proved by their dis- 

 posal in concentric zones, and by their dimensions. 

 These large crystals, thrown up in the lava completely 

 formed at the moment of its eruption, are mingled 

 either with the microliths or are contained in a 

 vitreous mass. The upheaval of the mass in which they 

 are held in suspension occurs subsequent to their com- 

 pleted development. The calm is succeeded by a period 

 of agitation, and the violently-ejected lava bruises, 

 corrodes, melts, or crushes the crystals. The microscope 

 shows us distinctly the phenomenon in question — the 

 large dislocated crystals with their fragments dispersed, 

 their edges blunted or worn away, their whole sub- 

 stance penetrated by the paste. 



Whilst the physical and chemical agents, started into 

 action by the movements of the lava, thus attack and 

 demolish the ancient crystals, the microliths are gradu- 

 ally forming. The vitreous mass in which the large 

 crystals are suspended, consists of numberless micro- 

 scopical particles. These are thus connected with a second 

 phase of crystallisation — they are developed in a moving 

 viscous magma.and their ulterior development is arrested 

 by a cooling of the lava so rapid as to cause almost in- 

 stantaneous solidification. 



Moreover, the arrangement of these microliths in 

 the lava clearly proves that this crystalline formation 

 was contemporary with the movements of the lava flow. 

 We notice at once in microscopical preparations that the 

 microliths accumulate round large crystallised 

 sections, and form undulating trails, thus presenting the 

 appearance known among micrographers as fluid structure. 

 It is caused by these infinitesimal acicular crystals 

 being arranged in one direction. When these trails of 

 microliths approach an enclosed crystal of large dimen- 

 sions, they flow round it, stream through the interstices 

 which separate larger sections, and show us the final 

 movement of the mass at the very moment of solidifica- 

 tion. 



The microscope proves, then, that crystallisation in lava 

 belongs to two periods. During the first, anterior to an 

 eruption, large crystals, already formed, are held in 

 suspension in a mass which we may suppose purely 

 vitreous. In the second, microliths and embryonic cry- 

 stalline forms appear, which date from the moment 

 of ejection or overflow, and are contemporary with the 

 consolidation of the rock. 



Microscopical observations on the crystals of the 

 second period point at once to the conclusion that they 

 were formed purely and simply by igneous process, 

 without taking into consideration either temperatures or 

 hypothetic pressures, two agents hitherto looked upon 

 as chief factors, and without claiming the absolute repose 

 which was one held indispensable for the regular crystal- 

 lisation of minerals. It is proved that the microliths are 

 formed after the eruption, at barometric pressure, at a 

 temperature by no means so high as was once supposed, 

 and the crystals are even known to form during the 

 actual movements of the lava stream. When the cool- 

 ing is very abrupt there is no time for microliths to 

 form, and the lava matter only produces crystallites. 



But the microscope enable us to determine the chrono- 

 logy of these lava crystals with even greater exactitude. 

 We have just remarked two great periods in their his- 

 tory ; let us see how we may, to a certain extent, deter- 

 mine the date at which each of the specimens of the two 

 groups became isolated from the glass. 



The particular circumstances which lead to this deter- 

 mination of their relative ages are their enclosures. 



A crystal which is developed from a vitreous mass 

 frequently accumulates particles from the medium in 

 which it is embedded. Thus certain sections appear, 

 under the microscope, riddled with vitreous grains 

 imprisoned in the heart of the crystal, and often arranged 

 according to the zones of development. These enclo- 

 sures sufficiently prove that that the crystals in question 

 are formed from a vitreous matter liquefied by heat. In 

 other cases, there are mineralogical specimens which are 

 found, in the form of microliths, in the interior of a crystal. 

 It is thus evident that these microliths are of earlier 

 origin than the mineral which imprisons them. Finally, 

 in yet other cases, sharply defined crystals are found 



