AOjfcKaO 



rfflO 



^1,0} fcvCaO. 



CaO 



FeA 



THE MUGGE DIAGRAM. 



In the former the relative quantities of oxide compo- 

 nents, expressed molecularly, are plotted on four lines in- 

 tersecting in angles of 45^. The silica is plotted on the 

 horizontal axis, half to the right and half to the left of the 

 origin. The other components are plotted on the other axes 

 as shown. It will be noticed that the oxides of iron are 

 plotted in the order FeO and FcjOg added. 



In the Milgge diagram the silica is divided equally into 

 eight parts, and plotted on the axes from the origin, thus 

 forming an octagon when joined up. The other components 

 are plotted outside this area on particular axes by adding 

 their values to the silica already plotted. The alumina is 

 divided into three parts — an equal molecular proportion to 

 potash on the left, an equal molecular proportion to soda 

 on the vertical, and the remainder plotted on the right hori- 

 zontal for lime. 



IV. — Slasfgy and Vitrophyric Types of Lava. 



In the north-western corner of Brown Lake there is an 

 extensive outflow of slaggy and ropy lava. The nature of 

 the mass varies from a very fine visicular type to a coarse 



