CLASSIFICATION OF IGNEOUS ROCKS 271 



responsible for much ; but the following table, giving the varying 

 percentage of the different chemical constituents, compiled from 

 Dr. Washington's Analyses, shows that the new classification is 

 also at fault : 



SiO., 



Ai.,o 3 



Fe.,0 3 



Feb 



MgO 



CaO 



K,0 



Na,0 



KO". + Na,0 



77 - 6i to 66'28 

 18*41 to 1C84 

 5 - 3i to nil 

 476 to nil 

 0*84 to nil 

 1*42 to trace 

 677 to 370 

 567 to 2-56 



This shows a wide range for what is, after all, a very minor 

 subdivision of the classification. The silica has a variation of 

 more than 11 per cent, and the other constituents show wide 

 variations. 



Other sub-rangs give a similar result. Toscanose, the sodi- 

 potassic sub-rang of the domalkalic rang of the quardofelic order 

 of the persalanes, includes rocks variously described as granites, 

 syenites, and diorites, besides their hemi-crystalline and glassy 

 representatives, and has a range of silica from 76*48 to 62'20, 

 alumina from 20*82 to i2'04, potash from 7*09 to 3*28, and the 

 alkali molecules from -141 to "079. 



Andose, the dosodic sub-rang of the alkalicalcic rang of the 

 perfelic order of the dosalanes, has a percentage of silica varying 

 from 58*20 to 44/85, and includes rocks whose descriptions range 

 from tonalite and quartz diorite to gabbro. 



These examples are sufficient to demonstrate that these small 

 groups have individually no very definite chemical significance. 



If we turn to the larger divisions, we find that they also do 

 not correspond to any clearly characterised variations in chemical 

 composition. 



The class persalane includes 1 a quartz porphyry with a silica 

 percentage of 80*99 an d an anorthosite with one of 4578. The 

 extreme limits are, however, much farther apart, and would 

 include pegmatites with over 85 per cent, and intrusive quartz 

 veins with practically 100 per cent, of silica. 2 In the other 

 direction, the analysis of a rock consisting mainly of corundum, 

 spinel, and anorthite, described by Morozevicz 3 under the 



1 Washington, op. cit., pp. 122, 206. 



2 Iddings, op. cit., p. 64. 



3 Tscherm. A/in. Pet. Mitth., vol. xviii. (1898) p. 212 ; Washington, op. cit., p. 217. 



