THE ORIGIN AND DESCENT OF ROCKS. 457 



PO 5 3 



III. -^rr<T>F> pyroxene or olivine, equal or nearly equal to the mitic 



Moo 



minerals, polmitic. 



PO 3 1 



IV. — <^>=-, dominant mitic minerals, domitic. 



PO 1 

 V. -TV- < =-, extremely rich in mitic minerals, permitic. 



In the first three orders a distinction between pyroxene and olivine is recog- 

 nized by sections, five in number: 



P 7 

 Section 1. 7a>^, extremely rich in pyroxene, perpyric. 



P 7 5 . 



2. ?\<T">o'j dominant pyroxene, dopyric. 



P 5 3 



3. ri<75->F) pyroxene and olivine, equal or nearly equal, pyrolic. 



P 3 1 



4. f\<F>7"> dominant olivine, domolic. 



PI 



5. 7\<Y) extremely rich in olivine, perolic. 



In the last two orders a distinction between the preponderant mitic minerals 

 is recognized by suborders, five in number. The minerals containing Fe^O, are 

 compared with those containing TiO.,. The former, magnetite and hematite, 

 are called hemic, mnemonic of hematite; the latter subgroup, titanite, ilmenite, 

 perofskite, rutile, are called tilic, mnemonic of titanite and ilmenite. Of orders 

 4 and 5, there are 



TT y 



Suborder 1. q^>T', hemic minerals extreme, perhemic. 



TT 7 5 



2. Tf^<rr->iT, dominant hemic minerals, dohemic. 

 i 1 <j 



TT pj q 



3. ^ <3~>F? hemic and tilic minerals equal or nearly equal, tilhemic. 



TT O 1 



4. ™<-^>=-, dominant tilic minerals, dotilic. 



TT 1 



5. m < y, tilic minerals extreme, pertilic. 



Further subdivision, producing rangs and subrangs, is made on the character 

 of the chemical bases in the standard minerals used in forming orders and is ex- 

 pressed in terms of the molecular proportions of certain oxides. For the salic 

 minerals, forming orders in the first three classes, the bases are alkalies — KjO 

 and Na20 — and lime, CaO. For the femic minerals, forming orders in the last 

 two classes, the bases are MgO, FeO, CaO and alkalies, K2O, Na^O. In classes 

 I, II, and III rangs are formed by comparing salic. alkalies, K2O' +Na20', with 

 salic lime, CaO'; and subrangs are formed by comparing K2O' with NajO'. 



