6 GEOLOGICAL MEMOIRS. 



the inverse takes place, and the quantity of these carhonates 

 diminishes. 



But is it permissible to conclude from these facts that whenever an 

 eruptive rock contains water it has been thereby decomposed ? I 

 think not, and I regret to find myself in disagreement on this point 

 with some eminent French and German geologists. 



§ 13. An eruptive rock has most commonly a com/plicated origin. — 

 The study of volcanic rocks shows that the definition of igneous or 

 aqueous rocks cannot be exact. It is necessary to give to these words 

 a different signification from those which they have in ordinary 

 language. Thus, when we say of a rock that it had an igneous 

 origin, it is not meant that it had been brought to a state of fusion 

 by heat alone : similarly of a rock of aqueous origin, that it will 

 only become plastic by the action of water. It must be understood 

 that these expressions have regard only to the principal agent in the 

 formation. 



I repeat, that although heat or water may well play a predomi- 

 nant part in the formation of an eruptive rock, in no case is this 

 agency exclusive ; it is accompanied by other agents, especially by 

 pressure, and in general by the very various forces which can incite 

 molecular action. 



Rocks assuredly present very different characters according to the 

 causes which have presided at their formation; but when these 

 causes are multiple, the characters become intermediate and much 

 less defined. The principal causes — heat, water, and pressure — are 

 usually found combined. 



It is then very certain that an eruptive rock can have a mixed 

 and very complex origin. Is it then astonishing after this, that 

 interminable discussions have been raised amongst geologists when 

 they have tried to explain the origin of rocks ? and is it not evident 

 that the problem was not always to be solved on the terms within 

 which it was restricted ? 



§ 14. Eruptive Bocks. — The preceding general considerations per- 

 mit us now to examine the origin of different rocks. 



Heat imprinting its peculiar mark upon those rocks which it has 

 contributed to form, it is natural to group rocks according to the 

 importance of the part it has played. They may be thus divided 

 into three classes, according as they are igneous, pseudo-igneous, or 

 non-igneous. 



I. Igneous Bocks. 



§ 15. The igneous rocks have been reduced to a state of fusion, or 

 at least rendered plastic, by heat. They are almost always anhydrous. 

 They have a cellular structure, and to a certain degree are rough to 

 the touch. The minerals they contain have a decided and character- 

 istic vitreous glance. They form the rocks which are considered as 

 eminently volcanic ; they often occur as lavas, and have preserved 

 traces of fusion. 



Their extreme types are trachyte and dolerite, which "represent all 

 their qualities. 



