ROCK SYNTHESIS The methods of investigation described above may be grouped together as analytical, in contradistinction to the synthetic in vestigation of rocks—which proceeds by experimental work to reproduce different rock types and thus to elucidate their origin and explain their structures. Though the experiments of de Saussure (174o-99) on the fusion of granites and porphyries may be said to mark the earliest beginnings of experimental petrology, the era of rock synthesis must be considered to date from the time of Sir James Hall's researches on the fusion of dolerites found in the neighbourhood of Edinburgh. This investigator showed convincingly that the crystalline dolerites (or whin stones) could be fused and consolidated, according to the rate of cooling, either as black glasses resembling natural pitchstones or as crystalline aggregates of minerals much like the dolerites from which they came. Hall's most famous experiments however, were conducted with limestones. Powdered chalk by being heated in gas-tight gun barrels, was converted into a crystalline mass of calcite, thus supporting the contention of Hutton that heat and pressure had consolidated limestones and converted them into marbles. A lapse of almost 90 years occurred before the experi ments of Hall were substantiated, when in 1878 the French petrologists Fouque and Michel Levy began their extensive researches on the synthesis of minerals and rocks by pyrogenetic methods. They succeeded in producing, by the use of a gas furnace and a nitrogen thermometer such rocks as porphyrite, basalt and dolerite, at the same time obtaining the characteristic textures—porphyritic, ophitic, etc.—by modifying the conditions under which the melts were cooled. With the more siliceous or acid rocks their experiments were much less successful. They advanced for the first time in a convincing manner the explana tion that for the crystallization of these rocks the gases, never absent in natural rock magmas, were indispensable mineralizing agents (mineralizers). It is now known that the formation of many minerals is facilitated, or can only be accomplished, in the presence of volatile constituents, as water, borates, chlorides, fluorides, etc. Not only do they assist in promoting the fluidity of the liquid and facilitate crystallization, but they form essen tial constituents of some of the important minerals occurring in acid igneous rocks (micas, tourmaline, topaz, etc.).
Among the pioneers in synthetic petrology may also be men tioned Ste. Clair Deville, Senarmont, Berthier, Bourgois, Haute feuille, von Chrustschoff, Doelter, Morosewicz and Vogt. To Vogt we owe the first comprehensive essay towards bringing the crystallization of igneous rock magmas definitely under the known laws of solution. Beginning a study of a large series of silicate slags, more or less comparable in composition with igneous rocks, he has brought together a large, body of information throwing light on the order of crystallization, the composition of the eutectics and the lowering of freezing points of the minerals in slag mixtures, and has directly applied these results to the elucida tion of the crystallization processes in natural magmas.
Up to this period the synthetic work, involving as it did the measurement of the melting point of minerals and the succession of crystallization in more or less complex mixtures, though very suggestive, lacked the strictly quantitative element. Experi
ments were carried out on materials always containing impuri ties or foreign substances, and the methods adopted for the deter mination of fixed thermal points as melting points, inversion points or heat changes were of doubtful precision. Since 1904, with the establishment of the Geophysical Laboratory at Wash ington, synthetic or physico-chemical petrology has entered upon a new quantitative era. The work of this laboratory was under taken to enter upon a quantitative study of rock formation which should include both the minerals and rocks which are geologically important and those which are economically useful, those formed directly from the magma and those formed by subsequent altera tion. The individual problems are in reality problems for physics and physical chemistry; but the delay in their attack has lain in the fact that the measured relations established by the exact sciences have scarcely been of adequate scope to meet the needs of large petrological questions. The great body of physical and physico-chemical measurements have been confined to a tempera ture region below ioo° C, while processes operating in rocks may have extended over a temperature region extending to 1,40o°, an enormous range over which to stretch the application of ordinary methods and one in which the common forms of ap paratus will not only fail but are themselves threatened with destruction. The initial work of the Geophysical Laboratory has been primarily to extend the methods of accurate temperature measurement to include the entire field of rock formation from o° to 1,600° ; the electric pyrometer has reached now such pre cision that an error of one or two degrees is all that need be expected in measuring temperatures up to 1,60o°; moreover these temperatures can be maintained quite steady for days or weeks at a time. Calorimetric measurements have been improved; spe cific heats can be determined with great accuracy even at the highest temperatures. At the same time petrographic methods have been advanced so that the crystallographic and optical con stants of the very minute crystals obtained in silicate melts can be accurately measured. The investigation of mineral substances under high pressures and high temperatures combined, and in the presence of volatile constituents has also been a subject of study. With these methods of precision available for the study of rock materials, examination of simple mineralogical systems has been prosecuted. An understanding of the chemistry of the common oxides of rocks and their combinations is essential to the progress of petrology. Already experimental work has been conducted on the combinations of the oxides CaO, MgO, A1203, ; the six possible binary systems have been fully worked out and the four possible ternary systems thoroughly studied, while portions of the quaternary system have been successfully explored. Much light has thereby been thrown on the processes taking place during the consolidation of igneous magmas which, though much more com plex in their constitution, yet clearly show by petrographic ex amination closely similar phenomena, as are revealed in the crystallization behaviour of these simpler systems. The applica tion of these laboratory investigations to elucidate the origin and evolution of igneous rock types will be considered hereafter under the heading of physical chemistry of igneous magmas.