VOLCANIC NECKS OF HORNSBY AND DCNDAS. 533 



tain ferric iron ; the picotite of this rock as calculated 



would have the following composition, A1 2 3 67*2 



FeO 41 



MgO 24-7 



Or 2 O a 4'0 



100*0 



This mineral lias previously been recorded aschromite, but 

 the constant low percentage of chromium through the rocks 

 studied, dunite, diallagite and quartz-carbonate rock, and 

 the considerable amount of alumina in the first and last of 

 these agree in referring the mineral to picotite. It has 

 been shown frequently that the two minerals are but the 

 end points of a series joined by a great number of specimens 

 of intermediate composition. These have recently been 

 discussed by Harker, 1 and Pratt and Lewis. 2 

 Altered Peridotites. 



Serpentine.— A beautiful example of olivine serpentine 

 occurs with typical mesh structure. (Plate 31, fig. 4.) The 

 cracks in the olivine are filled with magnetite and dusty 

 carbonates, while the meshes are lined with colourless 

 serpentine in the centre of which is often a rounded iso- 

 tropic grain of very low refractive index — this is opal. 

 Residual olivine grains also occur. A little green faintly 

 pleochroic chlorite of birefringence greater than that of 

 the serpentine is also present. A few unaltered grains 

 of diallage are scattered about and a number of dark 

 brovvn grains of picotite. 



Another very interesting alteration is shown in a harz- 

 burgite vein in a dunite. The pyroxene (enstatite) is passing 

 into a pleochroic red-brown fibrous mineral of high birefrin. 

 gence, which seems to be anthophyllite. The olivine has 



1 Tertiary Igneous Rocks of Skye, 1904, p. 70. 



■ On Corundum and the Peridotites of Western North Carolina, Rep. 



