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SCIENTIFIC NEWS. 



[July 27, 1888. 



us to suppose that the ancient stratified rocks were 

 formed under identical conditions, and we accept them as 

 sub-marine or fluvial deposits. Thus water is the main 

 agent in the formation of sedimentary and detrital 

 masses. 



The second group, to which we will now give our 

 particular attention, includes the rocky masses. Types 

 of this group we can observe in process of formation 

 during volcanic manifestations. The molten matter 

 ejected by the crater, or injected into the sedimentary 

 layers, solidifies as it cools. The minerals which consti- 

 tute the lavas are individual crystals developed in the 

 molten magma in which they are contained. These 

 crystals present no detritus in the sense that we now 

 attach to this word. Broadly speaking, these eruptive 

 masses do not show the stratiform tendency of the sedi- 

 mentary formations ; instead of the primitive horizon- 

 tality and the regular superposition of the stratified 

 layers, we have in the lavas an aspect which clearly 

 proves the forces of upheaval from below to which they 

 are subjected at the time of their eruption. Finally, 

 these massive rocks bear no traces of animal organisation. 



Let us compare, in their turn, the contemporary vol- 

 canic products with certain ancient crystalline rocks — 

 the granites, porphyries, trachytes, basalts. We notice 

 that all these latter possess close analogies of structure 

 and composition with the products of modern volcanic 

 activity. From this resemblance of their common cha- 

 racteristic features, we may conclude that the massive 

 rocks which traverse the sedimentary geological forma- 

 tion were, at first, like the modern lavas, forcibly 

 upheaved from below, and that they share with these 

 lavas an eruptive origin. 



But while we may see the sedimentary rocks formed 

 almost before our eyes, the conditions which influence 

 their formation, carried out, as it were, in broad daylight, 

 the eruptive masses are elaborated in the innermost 

 recesses of the earth. Their genesis is, to a certain ex- 

 tent, surrounded by mystery ; the eye cannot penetrate 

 into the vast subterranean reservoirs where these molten 

 masses first solidify, and whence they are ejected at 

 periods of volcanic activity. Here the process of direct 

 reasoning is rendered practically impossible ; the finest 

 analysis, the closest reasonings cannot replace the miss- 

 ing data — they are powerless to fully enlighten us as to 

 all the causes at work in the formation of eruptive rocks. 



To resolve our doubts, control and complete our ob- 

 servations, we must have resource to an artificial repro- 

 duction of volcanic rocks, and thence form a synthesis. 

 Armed with the data of such observations as we have 

 to act as guide, we endeavour by scientific manipulation 

 to imitate the results of nature. The science of the 

 earth, hitherto analytical, thus enters on a new phase 

 and becomes synthetical. Although restricted by the avail- 

 able apparatus, these endeavours to imitate nature, ori- 

 ginated and conducted by man's intelligence, enable us 

 to produce results analogous to those which we wish to 

 investigate, to direct and watch over the progress of the 

 phenomena, to obtain an exact account of their relations 

 to each other, and to vary at will the conditions under 

 which they arise. The knowledge acquired by obser- 

 vation, the analysis, and the reduction are thus all, to 

 use Bacon of Verulam's expression, " submitted to the 

 iron and fire of experience." 



We have given, in broad outlines, three standpoints 

 in the onward march of the knowledge of the earth's 

 crust. We have seen it at its earliest stage, when its 



aim was purely utilitarian, we have followed it later ; 

 when, guided by observation and reason, it rose to the 

 dignity of a science. Geology, in its last phase, is, at 

 the present day, transformed into an experimental 

 science. 



The study of the recent artificial reproduction of 

 volcanic rocks amply proves the powerful assistance 

 rendered by laboratory research to observation taken 

 direct from nature. But before explaining the process 

 of the synthesis of contemporary eruptive rocks, we 

 must sum up all that analysis and the observation of 

 facts have taught us on the constitution and mode of forma- 

 tion of these volcanic masses. The point of comparison 

 of the synthesis is to be found in the natural lavas — 

 they are the models which we wish to reproduce, and 

 it is therefore necessary to thoroughly understand them 

 in order to successfully imitate them in minute details. 



Let us reconsider, then, what we know about these 

 lavas and the conditions which influence their forma- 

 tion, although we cannot pause here to contemplate 

 the great manifestations of the internal forces of the 

 globe, nor the trains of accompanying phenomena — the 

 formidable eruptions which shake the volcano to its 

 very foundation, and project pulverised vitreous masses 

 and red hot stones. In the middle of this disturbance 

 the crater and the flanks of the mountain, fissured 

 by the force of the upheaval, emit streams of molten 

 lava which roll down the slopes and slowly solidify. 



The chief feature of an eruption is the emission of 

 lava or stream of molten matter which escapes from the 

 crater. Broadly speaking, one may best compare this 

 lava to glass liquefied by the influence of the high 

 temperature which predominates under the solid crust 

 of the globe. Direct observations of the temperature of 

 the liquefied lava of the crater, made at the moment of 

 its eruption, are fraught with dangers such as few 

 observers care to face. Thus we possess on this point 

 only approximate indications, although on some volca- 

 noes where the expulsion of lava is never very energetic, 

 but which are permanently in a state of moderate 

 activity, as in the Island of Hawaii, it is possible for 

 intrepid scientists to approach sufficiently near to the 

 crater to attempt to estimate the temperature of the 

 liquid mass. They have thus been enabled to average it 

 as varying between i,ooo c and 2,ooo p . But from the 

 moment the lava begins to flow, the temperature of the 

 surface rapidly decreases ; the sheet of liquid is covered 

 with a crust of scoria?, beneath which the molten mass 

 flows on like a stream, at about the temperature for 

 melting steel. This mantle of scoriae prevents irradiation, 

 and enables the mass which it covers to preserve for 

 some time a certain amount of viscosity. 



A little later we will discuss the observations on the 

 phenomena of the crystallisation which occurs in these 

 erupted masses, still fluid and viscous, but on the verge 

 of instant solidification. Let us consider, first, some of 

 the essential characteristics of the structure and composi- 

 tion of the lava. These volcanic products are often full 

 of bubbles (scoriaceous) ; sometimes, on the contrary, 

 they appear as homogenous, glassy masses, more or 

 less dark in colour, in which the naked eye can detect 

 no isolated minerals. Sometimes, again, the mass is so 

 full of minerals that the glassy paste which unites them 

 is scarcely recognisable. The minerals thus imprisoned, 

 when their development is perfect, present regular 

 polyhedral forms, constant for each specimen — that is to 

 say, they are crystals, perfect individual specimens of 



