Geology of Nil! ah cootie. 293 



Under the niicrosoope the rock is seen to consist of quartz, 

 orthoolase, plajrioclase, biotite and muscovite, wath minor acces- 

 sories, such as apatite and magnetite in very small amount. 

 The quartz is clear and colourless, although in parts it is slightly- 

 clouded by the numerous bubbles scattei*ed irregularly through 

 it. TliG larger gi'ains show numerous cracks. Both orthoclase 

 and plagioclase are present, the latter being slightly the more 

 abundant. The orthoclase is very much decomposed, tihe alterar- 

 tion products being mainly kaolin and secondary mica. Carlsbad 

 twins are fairly common. The orthoclase shows a considerable 

 amount of microperthitic intergi'owth, and patches of almost 

 fresh secondary plagioclase occur in the orthoclase. Th.e extinc- 

 tion angles of these secondary felspars show that they are albite. 

 The plagioclase is also very much altered, and the decomposition 

 products in this case are kaolin, secondary mica and zoisite. 

 Many of the felspars are so much decomposed that it is impos- 

 sible to read the angles of extinction, but one which was much 

 fresher than the remainder gave angles conforming to andesine 

 havinir the composition Abj^.An-,. This particular crystal is of 

 great interest, as it not only shows twinning according tO' the 

 carlsbad and albite law, but also accordino- to the maneback and 

 possibly the pericline law. Th.e centre of the minei'al has been 

 replaced by a secondary felspar which is quite fresh and shows 

 traces of albite twinning. The ref. index of the secondary felspar 

 is shown by Becke's method to be higher than that of the 

 original mineral, and so belongs to the basic end of the plagio- 

 clases. A few of the plagioclases show traces of zoning, but the 

 range of difference in composition is very slight. The muscovite 

 is fresh and occurs in irregular flakes, showing high polarization 

 colours. The biotite is bleached and partially converted to 

 chlorite, with the separation out of iron oxide along the cleavage 

 flakes. Under the microscope the character of the intergroA\i;h 

 of the quartz and felspar is well seen. In part this intergro^^^;h is 

 of a grannphyrio type, and in part it seems to consist of rounded 

 blebs of quartz set in a largre felspar crystal. 



(B). — The Granite Porphyry. 



The gi'anite porphyry has a considerable range in texture, 

 merging in one direction into a porjjhyritic gi'anite, and in the 



