If the mineralogical constituents of igneous rocks be considered from the 

 oetiological point of view they may be classified as follows : 



(1) Those foreign to the rock mass. 



(2) Those formed prior to the final consolidation of the rock. 



(3) Those formed in connection with final consolidation. 



(4) Those formed by secondary actions during or subsequent to 



consolidation. 



It is no part of our present purpose to discuss the origin of igneous 

 magmas. They may be portions of the original fluid material of the earth 

 as some suppose ; or they may result from the fusion of sedimentary or other 

 rocks. Our object is to describe and explain, so far as possible, the characters 

 which igneous rocks possess and we take as the starting point for this purpose 

 a mass of molten material. Now igneous rocks occasionally contain minerals 

 and fragments of rocks which have clearly been derived from pre-existing 

 rocks. These are regarded as foreign to the rock-mass in which they occur, 

 although it must be remembered that they may in some cases represent the 

 residue remaining after the fusion of a considerable mass of pre-existing 

 rock. 



Minerals of the second and third classes in the above scheme of classifica- 

 tion are termed primary. The distinctions between the two classes is most 

 sharply marked in the case of the volcanic rocks with porphyritic texture. 

 The material which issues from a volcano during an eruption is very rarely in 

 a state of true igneous fusion. In the majority of cases it contains crystals 

 and fragments of crystals which have been formed before the lava reaches the 

 surface. Thus, basaltic lavas frequently contain olivine and sometimes also 

 felspar and augite (e.g. lavas of Etna) ; andesitic lavas contain felspar and one 

 or more of the ferro-magnesian minerals such as hornblende, augite, 

 enstatite, (1) or biotite ; rhyolitic lavas contain quartz and felspar. These 

 minerals are usually of considerable size and therefore easily recognised by 

 the naked eye after the rock has consolidated. They belong to the second 

 group in the above classification. 



As the lava solidifies at the surface a further development of crystals 

 takes place. These are usually smaller in size and frequently require the 

 microscope for their detection. They constitute the groundmass of the rock, 

 and belong to the third group in the above classification. 



In the instance referred to, the distinction between the first and second 

 periods of crystallisation is well marked ; but this is not always the case. 

 Masses of molten material may consolidate at considerable depths and without 

 any movement occurring after the process of crystal-building has commenced. 

 Consolidation under these circumstances would in general be a slow and gradual 

 process. It would not be interrupted by any abrupt change in the conditions, 

 such as that which takes place when a molten mass is erupted at the surface. 

 Certain granites, diorites and gabbros are supposed to have been formed under 



(1) The term enstatite is here used to cover the entire group of rhombic pyroxenes. 



