456 GEOLOGY. 



apatite and the remaining minerals of this group. Most known igneous rocks 

 fall into the first subclass of each class. 



The classes are further divided into orders according to the proportions of 

 certain minerals in the preponderant subgroups. Thus Classes I, II, and III 

 are each divided into nine orders on a basis of the proportions of quartz and the 

 feldspars, and of the feldspars and the feldspathoids, quartz and feldspathoids 

 not occurring together. The orders may be described in the same terms for 

 each of the first three classes as follows: 



Q 7 

 Order I. ^>7-, extremely rich in quartz, perquaric. 



arte. 



7 5 

 II. Tp<^>o"> quartz dominant over feldspar, doqu 



5 3 



III. :n^< — >-^, quartz and feldspar equal or nearly equal, quarfelic. 



3 1 



IV. ^ < — >;=-, feldspar dominant over quartz, quardofelic. 



V. — :^ — <y, extremely rich in feldspar, perfelic. 



L 3 1 

 VI. ^<7->— , feldspar dominant over feldspathoids (lenads), lendofelic, 



T P q 



VII. Tn<o'>F> feldspar and lenads equal or nearly equal, lenjelic. 



T (T r 



VIII. T^<^>T> lenads dominant over feldspars, dolenic. 

 r 1 o 



L 7 

 IX. T^>r; extremely rich in lenads, perlenic. 

 r 1 



In classes IV and V the preponderant minerals are femic, and in subclass 1 

 they are silicates, titanates, and ferrates, with hematite and rutile. These are 

 subdivided as follows: 



Silicates — pyroxenes and olivine with akermanite in one subgroup ; the other 

 minerals, magnetite, hematite, ilmenite, titanite, perofskite, rutile, in the second 

 subgroup. This first group is called polic, mnemonic of pyroxene and olivine; 

 the second group is called mitic, mnemonic of magnetite, ilmenite, titanite. 



There are five orders in each of these classes, as follows: 



PC 7 

 Order I. -rf->T', extremely rich in pyroxene or olivine, perpolic. 



II. -Tr<T>':7} dominant pyroxene or olivine, dopolic. 



iVl J- o 



