Geohxji/ oj Mooroodac. 99 



travelling stage or eye-piece micrometer to obtain a number of 

 traverses acres* a microsection, and has shown that the volumes 

 of the different minerals are proportional to the sums of their 

 intercepts or any line or lines dra^vn across the rock, if the 

 number of minerals traversed be sufficient. Applying this 

 method, it wa« found that, out of a total length of 1035 units, 

 the sums of the intercepts of the different minerals were as 

 follows : — 



The percentage volumes are .shown in tlu' second column : — 

 Plagioelase - - - 11 4 - 10 



Quartz - - - - 3.)5 - 29.47 

 Orthoclase - - - IDS - 19.13 



Biotite - - - . 113 - 10.91 



Hornblende ... 5 - 0.48 



Apatite (estimatedj - - 2.5 - 0.24 



100.23 

 The specific gravities of the minerals is taken to be as follow : — 

 Plagioclase (Abj, An,) - = 2.ti5 



Orthoclase - - - - = 2.55 



Quartz = 2.65 



Biotite^ - - - - ^ 2.99 



Hornblende - - - - = 3.28 



Apatite - - - - = 3.20 



Multiplying the percentage volumes of the minerals by their 



densities we ol)tain the proportions by weight whicli are then 



recalculated as percentages. 



Plagioclase 



Quartz 



Orthoclase 



Biotite 



Hornblende 



.Apatite 



1 The specific s;ravity of the Biotite was deteniiined from Hakes b.v immersing them in 

 Sonstadt's heavy liquid, and determining- by the Westphal balance the specific gravity of 

 the liquid in which they floated in any position. The composition of the Biotite is assumed 

 to be similar to that of C'hebarkul of similar specific gravity (see Dana's System of Miner- 

 alogy, p. 630), while ttie Hornblende is assumed to be similar to that from a Vesuvian 

 localitv. 



