PLUTONIC EOCKS OF GAEABAL HILL AND MEALL BEEAC. Ill 



bubbles appear to be scattered irregularly through the grains, not 

 grouped along secondary planes. 



Interstitial quartz is found in the more basic of the intermediate 

 rocks, and granular quartz in the acid rocks. Micro-pegmatite and 

 granular quartz are found in the veins of eurite. A small quantity 

 of micro-pegmatite is also found in some of the rocks of intermediate 

 composition. In this case it forms tufts on the margins of oligo- 

 clase, and the felspar associated with the quartz is twinned like that 

 of the main grain or crystal to which the tuft is attached. 



(b) Primary Accessories. — Apatite is absent from the ultra-basic 

 rocks, but occurs in all the other members of the series. "vV ell- 

 formed prisms terminated by the basal plane are not uncommon. 

 Sphene is present in two forms : as colourless grains zoning iron ores 

 in the augite-diorites, and as brown pleochroic crystals, giving the 

 lozenge-shaped sections due to the form {123}, in the tonalites and 

 granitites. Minute crystals of zircon are not uncommon in the 

 intermediate and acid rocks. They occur as short, doubly terminated 

 prisms, rich in crystal-faces. 



(c) Secondary Accessories. — Serpentine arises as a result of the 

 alteration of olivine. Epidote and chlorite have been formed in 

 connexion with the alteration of biotite, and possibly also horn- 

 blende., The felspars have been more or less altered in certain 

 rocks, and whenever the alteration-product is capable of resolution 

 it is seen to consist of minute interlacing scales of mica. 



The distribution of minerals in the rocks which have been 

 examined microscopicallv is represented in the following Table 

 (p. 112). 



The Table merely gives the qualitative composition of the 

 rocks examined. It does not indicate the relative abundance 

 of the different minerals. If this were done the evidence of a 

 transition from the ultra-basic to the acid rocks would be strongly 

 emphasized. Nevertheless, it must be borne in mind that the 

 area as a whole appears to be relatively poor in typical basic 

 rocks — that is, in rocks containing about 50 per cent, of silica. 

 Gabbros, which are so common in someplutonic areas, are here con- 

 spicuous by their absence. The basic rocks in this district are re- 

 presented by augite-diorites. It is instructive to compare the general 

 distribution of the different minerals in the different rocks. For 

 this purpose it is convenient to arrange the rocks in the order of 

 specific gravity, which, of course, corresponds very closely with that 

 of silica-percentage, if we make allowance for the reduction of 

 specific gravity consequent on serpentinization. Then, commencing 

 with the most basic rocks and proceeding through the series, we 

 observe that as olivine dies out pyroxenes increase in importance. 

 These in turn are replaced by hornblende and biotite. N'ext the 

 hornblende decreases relatively to the biotite, and finally, in the 

 eurite-veins, the ferro-magnesian silicates have in places entirely 

 disappeared. If we turn to the quartzo-felspathic constituents, and 

 consider their distribution in the same way, we see that plagioclase 

 first makes its appearance ; then follows orthoclase, and lastly 



