274 SCIENCE PROGRESS 



In the orders there is less variation in the silica percentage. 

 It is, however, still very considerable in many cases. In the 

 analyses of the rocks included in Canadare, the perfelic order of 

 persalanes, the percentage of silica varies from 68"oi to 45 78, and 

 the possible range is from nearly 73 to less than 38. In Germa- 

 nare, the corresponding order of the dosalanes, it varies from 

 64'46 to 42*92 ; and in Scotare, the dopolic order of the dofemanes, 

 from 53-05 to 33*84- 1 



The variation in the composition of the orders of the classes 

 which are divided according to the proportion of quartz to 

 felspars, and of felspars to lenads or felspathoids, is explained to 

 a large extent by the fact that the alkali felspars with six 

 molecules of silica, and anorthite with only two, have exactly 

 the same classificatory value. In addition to this, in those 

 orders where the lenads have their share in the classification, 

 the position of rocks rich in soda is thrown out of relation to 

 that of those rich in potash by the fact that in the former 

 nepheline, with only two molecules of silica, has the same value 

 as leucite, with four such molecules, has in the latter. 



In the more basic classes the division into orders is based 

 on the proportion of femic silicates to non-silicates which are 

 mainly oxides, and this depends largely on the state of oxidation 

 of the iron, which ought not to be relied on for classificatory 

 purposes. 



In all the classes, the varying amounts of alumina, which 

 transfer lime and silica one way or the other between femic 

 silicates and felspars, effect considerable and unreasonable 

 changes in the delimitation of the orders. 



In the rangs the silica percentage varies practically to the 

 same extent as in the orders, for the lines of division do not 

 directly depend on the amount of silica present. In the more 

 acid classes the limits of the rangs in each order are, as we have 

 seen, determined by the ratio of salic alkalies to salic lime, not 

 that of the total alkalies to the total lime, which may be widely 

 different. The distinction between total and salic alkali is of no 

 great importance, as it is only when the percentage of alkali is 

 very high or that of alumina exceptionally low that any alkali 

 passes over to form femic silicates ; but the salic lime depends 

 on the amount of anorthite, and that is determined by the 

 alumina which is left when the alkalies are satisfied. If this be 



1 See Washington, op. cit., and Iddings, Client. Comp., p. 56. 



