GEOLOGY OF ERUPTIVE AND ASSOCIATED ROCKS, POKOLBIN. 403 



Si0 2 



Ti0 2 



A1 2 3 



Fe 2 0, 



FeO * 



MnO 



MgO 



CaO 



K 2 



Na 2 



H a O + 



H 2 



CO, 



p„6, 



Per cent. 

 55-20 



M.olec. 

 propor. 

 920 



1-17 



15 



20-14 



197 



3-55 



22 



3-46 



48 



0-09 



2 



1-10 



28 



9-17 



164 



0-96 



]0 



4-80 



77 



- 1-25 





- o-io 





absent 





trace 





100-99 



Quartz 



Orthoclase 



Albite 



Anorthite 



Diopside 



Wollastonite 



Magnetite 



Ilmenite 



Water 



Per cent. 



.. 4-98 



.. 5-56 



.. 40-34 



.. 30-58 



.. 9-27 



e 1'51 



.. 5-10 



.. 2-28 



.. 1-36 



100-98 



Sal 



81-46 



Fern 18*16 1 r c 



Class ii (Dosalane) 

 Q _ 4-98 1_ 

 F ~ 76-48 < 7 



Order 5 (Germanare) 

 K a O+Na 2 0_ 87^ 3 

 ~ 164 



CaO 

 Ran, 

 K 2 Q 



Na„0 



<*>! 



• 4 (Hessase) 

 10 _1 



77 < 7 



Subrang 4 



Andesite, Drake's Hill. 



Dark-coloured fine-grained rock. Well-developed flow- 

 structure shown by the parallel orientation of slightly por- 

 phyritic felspar crystals. Very small amount of ferro- 

 magnesian mineral visible in hand specimen. Considerable 

 amount of variation from place to place, the chief points 

 of variation being the size of the felspars, the amount of 

 ferro-magnesian constituent and the degree of development 

 of flow-structure. 



Under the microscope it is hypocrystalline and porphyritic. 

 The base is mostly glassy. Felspar is present in two 

 generations ; the larger are idiomorphic and twinned after 

 the albite law; a number of symmetrical sections gave a 

 maximum extinction angle of 26° so the felspar is labra- 

 dorite (AbiAni). The smaller ones are fine needles and 

 they accentuate the flow-structure. There is a good deal 

 of magnetite present. Very little ferro-magnesian mineral 

 is present, but a certain amount of pale green chlorite is 

 developed, and this seems to be probably from the alter- 

 ation of ferro-magnesian minerals. In one example there 

 is a small amount of decomposed biotite present. 



Pitchstone, Portion 42, Parish Pokolbin. 

 In hand specimen a black vitreous rock, almost com- 

 pletely glassy, a few small crystals of felspar being visible. 



