M. C. Deville on the origin of Granite. 183 



is a very singular and complicated equilibrium, an idea of 

 which may in some measure be obtained if we represent it 

 by a certain number of indeterminate equations, for the solu- 

 tion of which one condition is wanting. But .this condition 

 must be sought for in the study of the phenomena of igneous 

 fusion. 



M. Rose justly recommends geologists not to advance, without 

 weighty and powerful motives, hypotheses which are in contra- 

 diction with the known laws of chemistry. But may not 

 geologists well recommend chemists, who bring to these difficult 

 questions the aid of their special knowledge, to submit their 

 solutions to the control of natural facts ? However ingenious 

 these solutions may be, whatever support they may find in 

 laboratory experiments, the first and indispensable condition of 

 their adoption by a naturalist would be their agreement with 

 observed facts. 



But there are fundamental analogies, which no geologist since 

 Hutton has refused to recognize, between the oldest granite and 

 the most recent eruptive rocks, the volcanic lavas. This will be 

 readily admitted by any one who has seen, or has even merely 

 read a description of a flow of Vesuvius or Etna, of the amphi- 

 genic or doleritic veins of the Somma or of the Val del Bove; 

 the trap-dykes and melaphyr of Scotland or of the Palatinate ; 

 the Elvan of Cornwall ; the quartziferous porphyry of Saxony or 

 of Morvan ; lastly, some of the granitic veins so frequently met 

 with in the latter places, and in innumerable others. 



It is true that these general analogies of formation, the 

 establishment of which was one of the triumphs of the end of 

 the last century, do not amount to an absolute similitude : the 

 mineral elements, for example, vary from one group of rocks to 

 another. The presence of quartz, especially, has not been observed 

 in any true flow of lava, but it exists in the trachytes of the 

 Siebengebirge and of Mont-Dore : I have found it in the dole- 

 ritic rocks of Guadaloupe ; some considerable tracts of this latter 

 island, and of Martinique, are covered with a reddish earthy crust 

 which is full of innumerable fragments of transparent quartz, 

 arising, like it, from the decomposition of the volcanic rocks. It 

 likewise occurs with Labradorite in the doleritic trachyte which 

 constitutes the cone of the Soufriere. And since the density of 

 this quartz is 265, we cannot attribute to it an origin analogous 

 to that of the opals of Hungary. Thus step by step we succeed 

 in finding quartz in formations which, by their age,by their nature 

 and mode of occurrence, are nearest to the lavas which have 

 flowed under our own eyes. It is impossible to avoid establishing 

 a comparison between the conditions under which are formed the 

 crystals an agglomeration of which constitutes the rock, more 



