2 GEOLOGICAL MEMOIRS. 



Even when the heat is not sufficient to render a rock completely 

 plastic, it may, however, permit the substances of which it is com- 

 posed to combine amongst themselves, and hence it determines the 

 development of minerals. 



When the different eruptive rocks are submitted to a strong heat, 

 expeiience teaches us that they are softened and often even liquified. 

 But in cooling they become vitrified, or at least they put on characters 

 very different from those they displayed at first. There is no excep- 

 tion in this respect, any more than there is with the volcanic rocks. 



Moreover, the heat necessary to melt them is higher than that of 

 lavas. Many of the eruptive rocks are only merely agglutinated 

 by an amount of heat which would be more than sufficient to reduce 

 all the lavas to a complete fusion. This is the case notably in 

 serpentine, and rocks rich in magnesia, as also in granite and rocks 

 formed essentially of orthose and quartz. 



But it is specially necessary in this preliminary study to call 

 attention to the characters presented by the rocks which have incon- 

 testably an igneous origin. These characters are in reality well 

 defined, and imprint on the volcanic rocks an indelible stamp. 



We distinguish first of all a cellular structure, which, whether more 

 or less visible to the eye, is always apparent when we examine a 

 volcanic rock with the lens, and is never completely wanting. This 

 may result either from a disengagement of gas, or it may be from a 

 contraction of the molten matter. 



We observe also that the minerals of volcanic rocks, and above all 

 of lavas, have generally a vitreous glance. They may be, moreover, 

 splintery, and traversed by a multitude of fissures. This may be 

 readily verified in the felspars, amphigene, peridote, augite, and 

 hornblende. 



Now, although lavas may sometimes be very crystalline, they are 

 generally much less so than the rocks which apparently have not 

 been brought into a state of fusion ; and even when they present 

 very evident tir ces of fusion, there is always to be distinguished a 

 residue of crystallization which envelopes their minerals, and forms 

 what is termed their paste. 



Lavas have otherwise characters which it is very difficult to define, 

 but which will not permit them to be confounded with any other 

 eruptive rocks. Of all the rocks, they are the most easily recog- 

 nized. Hence, when an eruptive rock presents the cellular structure 

 and traces of fusion, whilst at the same time its minerals have a 

 vitreous fracture, we must necessarily assign to it an igneous origin, 

 that is to say, that heat was the principal cause of its formation. 



I would further remark that the eruptive rocks require generally 

 a much greater heat than that of lavas, to be, not molten, but 

 simply softened ; that, on the other hand, rocks presenting in a very 

 evident manner traces of igneous fusion are somewhat rare in the 

 terrestrial crust ; consequently it is only under exceptional circum- 

 stances that heat has played a principal part in the formation of 

 eruptive rocks. 



It becomes then necessary to pass under review the other causes 



