4 GEOLOGICAL MEMOIRS. 



The upheaval of mountains, which has taken place sometimes on 

 a gigantic scale, and which has brought up to the surface rocks from 

 a very great depth, has above all developed enormous pressures. 



Whether the pressure may have been permanent or casual, it is 

 certain that it has changed the characters of the rocks upon which it 

 has been exercised ; it has altered their physical properties. On the 

 other hand it has produced similarities in contiguous rocks ; it has 

 permitted the play of molecular actions and the development of 

 minerals ; consequently it has modified also the mineralogical compo- 

 sition of rocks. 



In researches upon the origin of rocks, it is necessary, then, to take 

 it into account, and its effects are to be added to those of heat and 

 water. 



§ 8. Molecular actions. — The molecular actions have also contri- 

 buted, it may be, to form the rocks, or it may be to metamorphose 

 them. However, it is proper to' consider them only as secondary 

 causes ; for they appear to be principally brought into play by heat, 

 by water, and by pressure. Electricity itself, which accompanies 

 and incites molecular actions, appears to result from those primary 

 causes. 



It is the molecular actions which engender the minerals composing 

 the rocks. They are exercised moreover whether the mineral sub- 

 stances are in a gaseous, liquid, or solid state. 



§9.-4 similar mineral may have an aqueous or an igneous origin. 

 — Great importance has been attached, and rightly, to the artificial 

 production of minerals ; and by means of very ingenious methods it 

 has been attempted to make them crystallize with the characters 

 which they present in nature. 



It ought to be remarked, however, that it is not by any means 

 necessary to reproduce a mineral to be assured that it had an aqueous 

 or an igneous origin. Thus when minerals, such as quartz and cal- 

 cite, have crystallized in stratified rocks enclosing fossils which could 

 have lived only in water, one cannot refuse to them an aqueous 

 origin ; similarly minerals, such as augite and peridote, which are 

 developed in the lavas ejected now by active volcanos, have incon- 

 testably an igneous origin. 



Since the clever experiments of Sir James Hall, researches on the 

 artificial production of minerals have been greatly multiplied, and 

 often crowned with success. Amongst the savans who have under- 

 taken them of late, I may mention specially, Hausmann, Mitscher- 

 lich, Berthier, Euchs, von Leonhard, Becquerel, Ebelmen, de Senar- 

 mont, Daubree, G. Bischoff, "Wohler, Bunsen, Percy, Durocher, 

 Manross, B. Cotta, Charles Deville, Damour, Caron, and especially 

 Henry Deville. 



But the bearings of the results obtained have been frequently ex- 

 aggerated ; for from the fact that a mineral had been obtained by the 

 dry way, or by the wet, it has been generally concluded that it had an 

 exclusive mode of formation. 



It is easy to see, however, that a similar min eral may have had 

 at one time an aqueous, at another time an igneous origin. 



