68 ■ J. A. Dunn: 



soda variety. Microperthitic intergrowth of albite in orthoclase 

 is common. Brown biotite altering in places to chlorite is an 

 essential constituent. Accessories are apatite (occasionally in 

 large crystals), zircon, and rarely sphene. 



A determination of the silica percentage gave a result closely 

 approximating Daly's average of 69.92 for true granites — this 

 result is given later. The silica percentage is rather high for a 

 granodiorite, and it is thus very possible that some of the ap- 

 parent plagioclase felspars may be anorthoclase. 



Basic Segregation around Inclusion, Big Hill. — A dark-grey, 

 fine to medium-grained holocrystalline rock, consisting in the 

 hand-specimen of quartz^ felspar, and abundant biotite. Both in 

 texture and appearance, it is quite distinct from the surround- 

 ing granite. 



Microscopically the rock is seen to be much finer in texture 

 than the granite. There is a large increase in plagioclase, de- 

 crease in percentage of quartz, whilst orthoclase is not at all 

 common. There is also a slight increase in biotite, whilst apatite, 

 though still relatively abundant, never occurs other than as small 

 crystals. Other accessories are zircon, sphene, .and a little 

 magnetite in biotite. Biotite altered in part to chlorite. 



This rock is the equivalent of a typical granodiorite, the silica 

 percentage (see later) approximating closely to Daly's average 

 of 65.15 for granodiorites. 



Inclusion of Altered Sandstone, Big Hill. — A fine-grained, 

 light-buff coloured rock, containing a good deal of mica. 



Microscopically the section shows a granular quartz mosaic, 

 with occasional sub-angular grains of orthoclase and plagioclase. 

 Abundant biotite, generally occupying interstices between quartz 

 grains. Detrital zircon and apatite, whilst needles of apatite are 

 often included in the quartz grains. 



The rock is evidently a re-crystallized sandstone. 



There are two possible modes of formation of the basic 

 segregation open to discussion, explained by each of the two 

 hypotheses of magmatic differentiation postulated by Daly and 

 Bowen respectively. 



4. — Mechanics of Intpusion. 



(a) Accepting first Daly's hypothesis of magmatic stoping 

 combined with marginal assimilation^ we would picture the molten 

 igneous mass intruding its way up through the Ordovician by 



