4:62 SoientifiG Intelligence, 



What the authors propose is as follows : All igneous rocks are 

 classified on the basis of their chemical composition ; all rocks 

 having like chemical composition are grouped together. 



The definition of the chemical composition of a rock and of a 

 unit of classification is expressed in terms of certain minerals 

 capable of crystallizing from a magma of a given chemical com- 

 position, and the expression is quantitative. For this purpose the 

 rock-making minerals are divided into two groups, the one of the 

 more siliceous alkali-and calcic-aluminous ones, the other of the 

 ferro-magnesian minerals. The first group is called mnemonically 

 the salic group : the second one, the femic group. From this 

 category the aluminous augites and amphiboles and the micas are 

 excluded for reasons given. 



To completely classify a rock by this system its chemical com- 

 position must be know^n by chemical analysis or approximately 

 so by physical or microscopic optical methods indicated by the 

 authors. Rocks once determined become types by which similar 

 rocks may be approximately classified. 



Since a given magma may cr3'stallize into quite different min- 

 eral combinations according to the different conditions attending 

 its solidification, it is necessary to select a certain set of salic and 

 femic minerals as uniform standards of comparison. These are 

 the ones ordinarily formed, but aluminous augite and hornblende 

 and micas are excluded. In practice, the molecular composition 

 of a rock obtained from its chemical composition (determined as 

 mentioned above) is computed into amounts of these standard 

 minerals and its place in the system is then easily determined. 



The standard mineral composition of a rock is called its norm, 

 and this may be quite different from its actual mineral composi- 

 tion or mode. Methods are given for obtaining the latter and 

 indicating its relation to the former. 



On the relative proportions of these two groups of standard 

 minerals the rocks are divided into five Classes, accordingly as 

 one or the other of these two groups alone constitutes the norm 

 or is extremely abundant ; w^hether one or the other is dominant ; 

 or whether the two are present in about equal proportions. These 

 Classes are then divided into Orders on the relative proportions 

 of the minerals forming the predominant group in each case and 

 in the middle group on the relative proportion of the salic min- 

 erals. So in the preponderantly salic classes the orders are based 

 on the relative proportions of quartz, feldspars and feldspathoids. 



The Orders are divided into Jiangs on the chemical character 

 of the basic oxides in the minerals in the preponderant group in 

 each case ; thus if these were feidspathic, as to whether they are 

 alkalic, alkali-calcic or calcic. The lowest division or grad obtains 

 only in the three intermediate classes and results from the con- 

 sideration of the relative amounts of the minerals forming the 

 subordinate group in each case. Where necessary there are sub- 

 classes, suborders, subrangs and subgrads. 



Texture is considered of minor importance and is taken into 

 account after the chemical and mineral composition. 



