

41 



Si O, ... ... ... 72-24 Oxy-en = 37'51 



A1 2 3 ......... 14-92) 



Fc.0, ......... 1-63 I 



Fe ... ... ... -231 



Mn ... ... ... -32 



CaO ......... 1-68 



MgO ......... -86 



K 2 O ... ... 5-10 



Na 2 ... ... ... 3.51 j 



Oxygen ratio = 37'51 : 7'46 : 2'50 



or = 15 : 2-0 : 1 



r ,. , . ., . 7-46 + 2-50 -.- 



ihe oxygen quotient in this case= r ^ - =-2bo. 



o t "Ol 



Now, of all the common rock-forming silicates, orthoclase and albite 

 contain the highest proportion of silica. The oxygen-ratio in both these 

 minerals is 12 : 3 : 1, and the oxygen-quotient is '333. Any rock, therefore, 

 which has a lower oxygen- quotient than this must contain free silica. Any 

 rock which has a higher oxygen-quotient may or may not contain free silica. 

 An examination of the elaborate tables of rock analyses compiled by J. 

 ROTH (1) will show that the oxygen-quotient of igneous rocks varies from about 

 2 in the case of obsidians and liparites (rhyolites) to about '8 or '9 in the 

 case of the olivine-rocks. 



Under certain circumstances, when the oxygen-ratios of the entire rock 

 and of the individual constituents are known, the percentage composition of 

 the rock in minerals may be calculated. (2) 



Thus the composition of the Donegal granite above referred to is 



Quartz ... ... ... ... 30-63 per cent. 



Orthoclase... ... ... ... 24-33 



Oligoclase ... ... ... ... 41-88 



Biotite ... ... ... ... 3-16 



Although there is, of course, a close connection between the composition 

 of the rock as a whole and that of the minerals which enter into its 

 composition, this connection is not so close as might at first sight be supposed. 

 Rocks of diverse chemical composition are often composed of the same 

 minerals, and rocks composed of the same minerals often differ considerably 

 in chemical composition. Observation and experiment show that the mineral- 

 ogical composition of a rock is determined by two factors : (1) the chemical 

 composition of the magma and (2) the physical conditions under which the 

 individualisation of definite chemical compounds has been effected. Thus ; if 

 hornblende, having a certain chemical composition, be reduced to a perfectly 

 molten state by intense heat and then allowed to cool it will, under certain 



(1) Beitrage zur Petrographie der plutonischen Gesteien. 



(2) For particulars as to this method, see Dr. HAUGHTON'S paper on " Irish Granites." 

 Q.J.G.S.. vol. XVIII., 1862. 



