Enough has been said to indicate the nature of the subject, and it will be more 

 fully dealt with in describing the different rock types. 



The foregoing discussion leads us to the conclusion that geological age 

 should not receive direct expression in petrographical nomenclature. The 

 petrographer should define his rock types without direct reference to age, and 

 then describe the distribution of his types in time. He should follow in short 

 the method of the palaeontologist. A similar course should be adopted so far 

 as matters relating to origin and geographical distribution are concerned. 

 The classification of igneous rocks should therefore be based primarily on 

 facts of composition and texture. 



The chemical composition of an unaltered igneous rock, not containing 

 any foreign substances, represents in a considerable number of cases, though 

 not probably in all, the composition of the original magma by the consolidation 

 of which it has been produced. In a previous chapter it has been shown 

 that, if we except certain rocks which are abnormally rich in alkalies, 

 the relations between different rocks, so far as chemical characters are 

 concerned, may be expressed in a broad and general way by the law of 

 BrxsEx. It seems natural, therefore, that special stress should be laid on 

 chemical composition in formulating a classification, and that the law of 

 BUXSEN should be made the basis of the classification. We accordingly find 

 that modern authors recognise at least three important groups the basic, the 

 intermediate and the acid groups. Some authors add a fourth the ultra- 

 basic to include the peridotites. 



The mineralogical composition of a rock is, however, much more easily 

 determined than its chemical composition, and accordingly it is found more 

 convenient, in actual practice, to define rock-types by reference to their 

 mineralogical rather than to their chemical constituents. This is the method 

 which will be adopted in the present work for convenience of description. 

 It must, however, be remembered that although there is of necessity a close 

 connection between chemical and mineralogical composition, the latter 

 is not in all cases governed by the former. One and the same magma 

 cooling under different conditions may give rise to rocks differing somewhat 

 in mineralogical composition. 



Texture, as we have seen, stands in intimate relation to the physical 

 conditions under which consolidation is effected, and may therefore be very 

 conveniently utilised for the purpose of forming sub-divisions. Messrs. 

 FovQufi and Lfivv recognise two important types of texture the granitic 

 and the trachytic corresponding to the division of igneous rocks generally 

 adopted in this country into plutonic and volcanic. The granitic texture is 

 characterised by the absence of amorphous matter and the crystallitic and 

 microlitic products usually associated with it ; and by the fact that the constituents 

 mostly belong to one period of crystallisation. The trachytic texture is 

 characterised by the frequent occurrence of amorphous matter ; by a finely 

 crystalline, compact, or vitreous texture ; and by the fact that the crystalline con- 

 stituents, in most cases, belong to at least two strongly marked periods of consolida- 

 tion. There is, of course, no hard and fast line between the two types of 

 texture. They shade into each other by the most imperceptible gradations. 



