10 GEOLOGICAL MEMOIRS. 



had a less elevated temperature. Thus has resulted the ahsence of 

 peridote, the presence of a great quantity of carbonates and zeolites, 

 and, above all, that the metamorphism exerted was less energetic. 



On the other hand, as the trap was completely fluid, I am led to 

 believe that it formed at the moment of eruption a kind of mortar or 

 miry paste. 



It is probable that it then contained a much greater quantity of 

 water than the water of crystallization which it has retained : to this 

 water it owed its great fluidity. It was only when its crystalline 

 structure was developed that it became lithoid, and that it took on 

 thus its hardness as well as its cohesion. 



I would remark, however, that the veins of trap may very well be 

 more or less argillaceous. There are some even having all the charac- 

 ters of true clays ; it has been always considered that these had been 

 decomposed and changed into a kind of kaolin. But to me it seems 

 that the trap has been preserved also in the state of muddy paste ; 

 for the characters taken by this paste must necessarily depend much 

 on its chemical composition : consequently whenever it was rich in 

 alkalies, for example, it became felspathic and very hard ; whilst in 

 the contrary case it could not well be solidified, but would remain 

 always in the state in which it was erupted. 



III. NON-IGNEOTJS EKTJPTIVE ROCKS. 



§ 23. The igneous and pseudo-igneous rocks represent those which 

 we ordinarily term volcanic. The non-igneous rocks which we now 

 study correspond to the plutonic rocks of Lyell. Their component 

 minerals have no longer the vitreous glance peculiar to igneous rocks. 

 Their structure is no longer cellular, usually even it is very compact. 

 Lastly, they are not associated with volcanic rocks, and consequently 

 they ought to have altogether another origin. It is really water and 

 pressure which has rendered them plastic ; for heat has only played 

 a secondary part in their formation. 



§ 24. Granite. — After giving in detail his views of granite, the 

 author sums up : It seems to me that granite presents none of the 

 characters of igneous rocks. For the development of its minerals 

 it was sufficient that it formed a magma slightly plastic, the study 

 of the granites of some localities showing even that it could crystal- 

 lize in nearly a solid state. Water, assisted by pressure, has most 

 probably contributed in the most efficacious manner to render granite 

 plastic. Heat has likewise assisted ; but it was very moderate in 

 degree, and certainly much below redness. If we suppose the granite 

 to have attained a sufficient state of plasticity, it is evident that in 

 other respects the crystallization of its minerals has been determined 

 by chemical and molecular actions. 



§ 25. Bio rite.— Opposite the granitic rocks it is proper to put the 

 trappean rocks, in which the felspathic anorthose is associated with 

 mica and with hornblende, that is to say, with those minerals which 

 we find in granite. I shall consider specially diorite, which is one 

 of the most widely spread and most important rocks. 



After noticing its characters, the author says : " Diorite approaches 



