No. 3.] MACFARLANE — ON" CRYSTALLINE ROCKS. 261 



The number of minerals which enter into the constitution of 

 rocks is very sm:ill compared with the number of the mineral spe- 

 cies which are found described in the various treatises on minera- 

 logy. Of the latter there are upwards of six hundred, but the 

 great majority of these are rare minerals, occurring in veins or 

 cavities, and not entering: into the constitution of the rocks them- 

 selves. Tiie number of minerals which are found in original 

 rocks is still more limited, and if from it if we deduct the sparin^ily 

 occurring, or so-c.iUed accessorial constituents, the number is 

 reduced to twenty minerals, which may be c:illed the essential 

 constituents of original rocks. The following table gives their 

 names and the silica contents of the extreme acid and basic 

 varieties. 



Mineral. Pccentaje of Silica. 

 Quartz 100 



Orthoclase C9 —62.75 



Oligoclase 64 25— 50.28 



Labradorite 55.63—50 31 



Anoithite 47.63—42.01 



Lcucite 58.10—53 50 



Nepheline 45.31 — 43.50 



Potash mica 51.73 — 43 47 



Magnesia mica 44.63 — 36.17 



Hornblundu' 60.60—37.84 



Pyroxene 57.40—38.58 



Hypcrsthene 51.35 



Enstatite 56.91 



Diallage 53.71—40 12 



Olivine 44.67—36.30 



Magnetite 00.00 



The sep:iration of the minerals occurring in rocks into essential 

 and acces.sovial constituents originated with German litholo2;ists 

 and may perhaps be regarded as arbitrary. In characterising the 

 sixteen minerals just mentioned as essential constituents, we have 

 however, to some extent, been guided by their chemical constitu- 

 tion. In the preceding chapter silicic acid, alumina, peroxide 

 of iron, protoxide of iron, magnesia, lime, soda and potash were 

 indicated as the essential chemical constituents of rocks; and only 

 sucli minerals as contain these substances, and no others, as esseii- 

 tlal ingredients, have been admitted into the table. This mode 

 of selection may perhaps be considered as arbitrary as any other, 

 for it causes the exclusion of the mineral tourmaline, which some- 

 times appears to deserve the rank of au essential constituent. 



