276 



NA TURE 



[yan. 17, 1889 



lithological synthesis. Let us first explain the manipula- 

 tions for the reproduction of one of the rocks which plays 

 the principal part in the eruptions of Vesuvius— /f«^<?- 

 tephrite. This rock is composed of leucite, labrador- 

 felspar, and augite. 



A mixture is formed of silica, alumina, lime, ferric 

 oxide, potash, and soda, corresponding to one part of 

 augite, four of labrador-felspar, and eight of leucite. This 

 mixture is introduced into the crucible, and fused at a 

 glowing white heat to a homogeneous glass. After fusion, 

 the temperature is lowered, and the vitreous mass is ex- 

 posed for forty-eight hours to the temperature of fused 

 steel. During this first phase, crystals of leucite separ- 

 ate. They evidently correspond to the first period of 

 consolidation in eruptive rocks. 



The matter is then subjected during another forty-eight 

 hours to the temperature of fused copper. All the mass, 

 the residue from which the crystals of leucite first 

 separated, is now transformed into microliths of augite 

 and labrador-felspar, with octahedra of magnetite and 

 picotite. 



Let us now compare microscopic preparations of the 

 synthetical product of this double recidt with those of 

 the natural lava. Not only have the same minerals been 

 reproduced by this dry fusion, but the order gf their 

 appearance and the proportion of the constituent species 

 are identical ; and their analogy may be pursued even to 

 the details of the crystallographic forms. The leucite, in 

 large crystals, offers all the features of this mineral in the 

 Vesuvian lavas ; and around these crystals are grouped 

 the microlites of the second period — the augite and 

 labrador. Finally, as in the natural rock, the leucite 

 contains inclusions of magnetic iron-ore and picotite, 

 which are the most ancient minerals in the rock. 



As a second example, let us take the synthesis of basalt 

 — one of the most widely-spread types of rock in the 

 volcanic series, and one which, so far as its origin is 

 concerned, has been the subject of numerous hypotheses. 

 It is known that basalt is composed essentially of three 

 minerals — olivine, augite, and labrador-felspar. The 

 olivine in the natural rock appears in crystals of the 

 first consolidation. 



As in the case of the leucotephrite, so here, we form a 

 mixture of the chemical constituents, or of the powdered 

 minerals, corresponding to the mean composition of a 

 basalt rich in olivine. Such a mixture is composed of 

 three parts of olivine, two of augite, and three of labrador. 

 This is first transformed into a homogeneous black glass. 

 During forty-eight hours it is maintained at a white heat. 

 If, after this recuit at a high temperature, we examine a 

 thin section of the glass, we observe large crystals of 

 olivine. These are as yet embedded in a vitreous mass, 

 in which small octahedra of massicotite and picotite are 

 isolated, as also a few crystals of augite. 



It remains now to produce the microlites of the second 

 consolidation, by which the crystals of olivine developed 

 during the first phase ought to be surrounded. To pro- 

 duce these, the mass is maintained at a cherry-red heat for 

 forty-eight hours. After this recuit we obtain a paste 

 composed of microlites of labrador and augite, with 

 magnetite and a vitreous substance which is the residue 

 of the crystallization. We have therefore, in this second 

 phase, reproduced the microlithic structure. These mani- 

 pulations produce basalts which we can scarcely distin- 

 guish from the natural rocks, and thus a few grammes of 

 a substance skilfully manipulated furnish us with the 

 most convincing proof of the purely igneous formation 

 of this rock. 



We could go on explainng these remarkable .series of 

 experiments iDy MM. Fouqud and Levy, in the same way 

 as we have dealt with the two preceding syntheses. All 

 the contemporary eruptive rocks have thus been repro- 

 duced: andesites, labradorites, basalts, limburgites, nephe- 

 Knites, tephrites, leucite rocks, peridotites, and labradorites 



with ophitic structure. We will, however, confine our- 

 selves, as a last example, to those processes by which 

 they have succeeded in directly explaining, by means 

 of synthesis, the eruptive phenomena of the older periods 

 of the globe. 



There are certain ancient crystalline rocks, common in 

 the Pyrenees, which are known as ophites. The period 

 at which they were formed, and their mode of origin, had 

 not been definitively established, when in 1877 M. L^vy 

 showed that they were eruptive and that they exhibited 

 under the microscope a remarkable structure which he 

 termed the " ophitic structure," the felspar being sur- 

 rounded by very large plates of augite. It seemed, then, 

 that the ophitic rocks were igneous rocks, in which the 

 cooling had been more slow than in the ordinary rocks 

 of modern eruptions. It was therefore necessary, in 

 attempting to reproduce the ophitic type by synthesis, tO' 

 cause the augite to crystallize during a phase sharply 

 separated from that in which the felspar was reproduced ; 

 and, moreover, to give to the augite sufficient time to 

 crystallize in large plates. For this purpose, a mixture of 

 one part of anorthite and two of augite was submitted, 

 after fusion, to a first remit., in which it was maintained 

 for forty-eight hours at the melting-point of steel : under 

 these conditions the anorthite separated. A second 

 recuit of the same duration .as the first, but at the fusing- 

 point of copper, led to the crystallizat'on of the augite in 

 large plates, which were moulded round the felspathic 

 element, and to which were added small octahedra of 

 magnetite and picotite. By this remarkable synthesis 

 the eruptive origin of the ophites, and the cause of their 

 structure, were established beyond all doubt. 



It. is thus seen how synthesis succeeds in explaining 

 the genesis of rocks, and in settling those discussions 

 which until recently were rife with respect to the prin- 

 cipal crystalline types of modern date ; those relating,, 

 for example, to basalt — a rock in whose formation it was 

 argued that water played an important part. Now, the 

 broad conclusion to be drawn from these experiments is 

 that basalt, and, indeed, modern volcanic rocks in genera% 

 have been formed by a fusion purely igneous. 



But by the side of these magnificent results the savants 

 have had to record many fruitless experiments. It is 

 useful to recall these by way of example, as they serve to 

 indicate the paths to be avoided if we would attain, 

 success. These failures circumscribe the field of future" 

 experiment, and mark the limits within which hypotheses 

 should have play. They demonstrate, moreover, that the 

 rocks which we have not succeeded in forming synthe- 

 tically by our methods must have been formed under 

 . different conditions from those which prevail in the 

 formation of modern volcanic products. This conclusion' 

 to which observation and analysis had already pointed, 

 without, however, precisely defining the cause, is thus 

 confirmed by the failure of our synthetic researches. If 

 synthesis has succeeded in reproducing all kinds of lava 

 i from modern eruptions, it has failed toimitate those rocks 

 1 which are no longer formed in contemporary eruptions. 

 It may be said, generally, that up to the present time all 

 the acid rocks have withstood our synthetic efforts, and 

 those which contain among their constituent minerals,, 

 quartz, mica, orthoclase, and hornblende. 



The processes of Nature involve no occult forces, and 

 it may be that by combining those means which are 

 already at our disposal, and in modifying their application^ 

 we may be permitted to witness the production of those 

 rocks which have hitherto eluded our efforts. Such a 

 hope is based on the results already attained, which we 

 may regard as only the presage of others perhaps still 

 more surprising. The failures of the past prepare for the 

 conquests of the morrow. 



In this rapid review of the progress of lithological 

 synthesis, I have endeavoured to show the high scientific 

 value of the researches instituted in the geological labora- 



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