THE ORIGIN AND DESCENT OF ROCKS. 459 



5. " <Y, extremely calcic, percalcic. 



Subrangs distinguish bet^Yeen MgO and FeO, thus: 



MgO 7 , . 



Subrang 1. -^."^ >— , extremely magnesic, permagnesic. I 



7 5 



2. " < — >^, dominantly magnesic, c/omag'nmc. 



1 o 



r Q 



3. ' ' < ^ > — , equally magnesic and ferrous, magnesif err oils, 



3 1 



4. " < — >=-, dominantly ferrous, doferroics, 



o I 



5. " <--, extremely ferrous, perferrous. 



Finally a recognition of the character of the subordinate standard minerals 

 leads to further subdivisions known as grads and subgrads. They only occur in 

 classes II, III, and IV, because these are the only ones in which the subordinate 

 minerals are in notable amounts. Grads are formed in a manner similar to that 

 employed to produce orders. Thus grads in classes II and III correspond to 

 orders in class IV and the reverse. Subgrads are the same in form as rangs 

 when the difference in the treatment of salic and femic minerals is borne in mind. 

 The names given to these divisions, which in fact recognize only the character 

 of the magma, are derived from geographical localities and embrace many of 

 those already in use, except that the names of orders are taken from countries 

 or nations. Specific terminations indicate the place in the series of divisions: 



ane for class, one for subclass. 



are for order, ore for suborder. 



ase for rang, ose for subrang. 



ate for grad, ote for subgrad. 



This may be illustrated as follows: 



Class I. persalane, all rocks extremely salic. 



Order 4. hritannare, feldspar dominant over quartz, quardofelic. Many rocks 

 of granitic composition whether crystalline or glassy. 



Rang 1. liparase, peralkalic, rocks in which the potential feldspars are ex- 

 tremely alkalic, orthoclase, or albite. 



Subrang 2. Omeose, dopotassic, rocks in which the extremely alkali feldspars 

 are dominantly potassic, orthoclase, with subordinate albite. Examples of 

 omeose are: granite from Omeo, Victoria, Australia, and rhyolite from Silver 

 Cliff, Colorado. 



The presence of distinctive minerals not indicated in the standard mineral 

 composition of norm is expressed by qualifying the magmatic name by the name 

 of the distinctive mineral; as, a hornblende-monzonose. 



The precise texture of the rock is expressed by qualifying the magmatic name 

 byatextural adjective; as, a grano-monzonose, a vitro-monzonose, a phyro-mon- 

 zonose, etc. 



