CLASSIFICATION OF THE PLUTONIC ROCKS 245 



obtained by arranging them in the order of their silica contents. 

 If with this arrangement, a division into series according to the 

 alkali and calc-alkali contents, as shown by the constituent 

 felspars and felspathoids, be combined, a serviceable system of 

 rock classification is obtained. 



Arranged in the order of their silica contents, the plutonic 

 rocks can be divided into three groups : 



(1) Acid, with silica contents above 66 per cent. ; 



(2) Intermediate, with silica contents between 66 and 52 per 

 cent. ; and 



(3) Basic, with silica contents below 52. 



These limits are, of course, quite arbitrary, but it will be 

 seen later that they permit a convenient separation in accordance 

 with existing records of rock types. At the extreme basic end 

 there is a small assemblage of rocks which contain very little or 

 no felspar. These rocks (the picrites, peridotites, hornblendites, 

 and pyroxenites) consist of ferromagnesian minerals and iron- 

 ores. They form a sub-group generally known as the Ultra- 

 basic rocks, which need not be considered here in the general 

 scheme of classification. 



The division according to alkali and calc-alkali contents is 

 accomplished by means of the felspars and felspathoids, the 

 variation in which facilitates the formation of a number of series, 

 each series being characterised by its dominant felspar or 

 felspathoid and each comprising acid, intermediate, and basic 

 rocks. Thus, a series can be formed which is characterised by 

 the predominance of the lime-bearing felspars (oligoclase to 

 anorthite). This may be conveniently termed the calc- 

 alkali series, and comprises rocks ranging from granodiorite 

 through tonalite and diorite to gabbro, according to the per- 

 centage of silica. In contrast with this is the alkali series, in 

 which there is a dominant alkali-felspar (orthoclase, micro- 

 perthite, soda-orthoclase, microcline, anorthoclase, albite, etc.), 

 or by a felspathoid (nepheline, sodalite, etc.). Occupying a 

 middle position between the calc-alkali and the alkali series is 

 the monzonite series, which comprises rock types characterised 

 by the presence of both orthoclase and a lime-bearing plagioclase, 

 without either being greatly predominant. 



The alkali series may be further sub-divided into : 



(1) A potash series with predominant potash-felspar (ortho- 

 clase, microline). 



