178 METAMORPHISM. 



substances themselves become more crystalline in character, and sometimes, 

 without losing the organic remains they contain, become filled with new 

 minerals ; in Brittany these schists are filled with andalu'site, sometimes 

 staurotides, near all granitic deposits. Elsewhere, as in Vosges, in the 

 mountains of Var, we see them pass to mica-schist ; and the latter to gneiss, 

 which, itself, insensibly becomes granite. Now, as if the intimate union 

 observed were not sufficient, these mica-shists, then the gneiss itself, contain 

 carburetted sch'st. or even graphite, veins of anthracite, which remind us of 

 the deposits which are found further in the schists of grauwackes, and suffi- 

 cintly marked to determine the pursuit of coal. 



It is, then, evident that all the rocks we have cited, no matter how they 

 may differ, are only modifications, mere metamorphoses of one or all ; and, 

 as it is in approaching granitic rocks, evidently produced by igneous action, 

 that these metamorphoses become more and more marked, it is clear that it 

 is to the influence of the latter that they are due. The same influence is 

 manifest on the sandstones of different ages, at various points where they 

 are in immediate contact with granite: the modifications are such that the 

 special name, arkose, has been applied to them. They then pass through 

 all shades to granite, and become filled with different substances that they 

 do not contain elsewhere. 



Near porphyritic ejections, schists frequently present modifications of an- 

 other kind. Here the most earthy, and the most evidently sedimentary parts, 

 pass by degrees to compact substances, more and more feldspathic, preserv- 

 ing more or less of their schistose character, and finally end by containing 

 crystals of feldspar ; elsewhere these same matters pass to solid clays, con- 

 taining veins of limestone, then nodules of the same substance, which as- 

 sume all the characters of amygdaloids, losing, only little by little, their 

 schistose structure. 



The same phenomena are remarked between diverse sandstones and por- 

 phyries that intersect them. The arena'ceous matter gradually hardens, 

 becomes more compact, and finally unites with the porphyry in such a 

 manner that it is not easy to determine where one begins or the other ends. 



All these facts pertain really, with the exception of some details, to ancient 

 jeology ; and it is only the manner of explaining them that has changed. 

 Everything conspiring to demonstrate that crystalline substances have been 

 produced by the action of fire, and forced through sedimentary deposits, we 

 now understand that the latter have been modified, or metamorphosed in 

 different wi;ys by their influence, in a degree corresponding to their proxi. 

 mity : the effects entirely cease only at greater or less distances. 



It is conceived that one part of these metamorphoses of sedimentary forma- 

 tions arises from the simple action of heat without new fusion, but sufficient 

 to modify the texture of masses, and even to unite elements in other propor- 

 tions, as happens when transparent glass is submitted to a temperature in- 

 sufficient to melt it, in which, nevertheless, a new crystallization takes place. 

 But this idea is not sufficient of itself; we must conceive another action, 

 which we are not yet able to explain or account for, in virtue of which par- 

 ticular substances have been borne, or developed, in the midst of rocks found 

 in the neighbourhood of divers upturnings, of which the globe is the theatre. 

 We readily conceive of the introduction of sulphuric acid, which is frequently 

 formed in volcanoes; but we do not understand thai of magnesia and diffe- 

 rent species of si'licates, and, as respects them, all is still purely hypothetical. 

 We may compare these facts to cementa'tion, by means of which iron is 

 converted into steel ; a phenomenon which is manifested not only in contact 

 with carbona'ccous matter, but extends far into the ferru'ginous mass, and 

 even takes place at a distance, according to the experiments of M. Laurent, 

 wno hat- shown that carbona'ceous matter may penetrate iron even through 



