THE CLASSIFICATION OF IGNEOUS ROCKS. 477 



originally vitreous. Whether this idea has in some cases 

 been pushed too far is a point that might be argued, but it 

 is certain that the Continental workers as a body have not 

 yet fully appreciated its importance. The extent to which 

 such secondary changes may obscure the true nature of the 

 rocks is well illustrated by the ancient lavas of the Eastern 

 States of America. Recent papers, published in that coun- 

 try, have fully recognised the importance of devitrification 

 and other transformations, and the altered rocks have been 

 designated by such terms as aporhyolite, apodacite, etc., to 

 mark at once their initial characters and the fact of their 

 subsequent alteration. 



There is a sense in which the geological relations or 

 mode of occurrence of a rock may enter into petrological 

 classification. On the one hand, as marking the conditions 

 under which consolidation took place, the mode of occurrence 

 is connected with genetic considerations ; on the other hand 

 it is in close relation with the structure and texture of the 

 rock, as indeed is implied in the use of such terms as 

 " plutonic ' : and " volcanic ' to denote divisions of rocks 

 which are really defined by structural characters. 



We ousfht next to consider to what extent the cJiemical 

 composition of igneous rocks may enter, directly or indirectly, 

 into their systematic arrangement. It is clear that if we are 

 at liberty to regard each rock as representing the result of 

 consolidation of a given molten magma, the composition of 

 that magma may be proposed with considerable cogency as 

 a prime factor in petrological classification. The empirical 

 laws which seem to hold with a high degree of generality 

 in the chemical analyses of igneous rocks, pave the way for 

 such a system. The percentages of the several oxides are 

 found to vary within more or less defined limits, and their 

 variations are in great measure correlated, so that with an 

 increase in the amount of silica we find generally an increase 

 in certain other oxides and a diminution in others. Thus a 

 large proportion of igneous rocks fall roughly into a linear 

 series, having at the "acid" end types rich in silica and 

 alkalies, at the " basic " end others rich in magnesia and 

 iron-oxides. The laws here roughly expressed, and subject 



