Th", Petro(jraphy of the Intrusive, Dio.hnse. 91 



Irregular grains of magnetite with sparse ones of titaniferous iron, 

 the former in places of considerable size, are present in all the sections 

 examined, but never in any marked quantity. 



{h.) Dykes and Sills. 



As a rule the dykes and sills consist of medium, fine-grained, grey 

 to dark-grey, compact diabase varying in specific gravity from 2-90 

 to 3-18. In hand-specimens the only mineral usually recognisable 

 is feldspar, which occurs in small white specks ; but specimens from the 

 inner parts of some of the dykes show structures approaching tliose 

 described as characterising the diabase of the mountain-masses, and in 

 them broad ophitic crystals and masses of pyroxene are clearly seen. 



The relative distribution of pyroxenes and the feldspars in the inner 

 and the outer parts of the dykes varies to a marked extent. In the 

 majority of them in the outer parts the pyroxenes and the iron-ores 

 are the preponderant minerals, whilst in the inner parts plagioclase, 

 with small irregular areas of a micro-pegmatite of feldspar and quartz, 

 forms by far the more abundant constituent. In other dykes the 

 reverse is the case, the ferro-magnesian constituents being concenti'ated 

 towards the middle of the mass ; whilst in manj^ there is little difierence 

 between the inner and the outer parts in this respect. 



Where the inner i^arts are the more feldspathic they are of medium 

 to somewhat coarse texture, and are lighter in colour and of lower 

 specific gravity than are the outer parts. In thin sections of these the 

 inner parts are seen to be made up of many narrow with some broad 

 lath-like crystals of labradorite, the larger ones being in places somewhat 

 cloudy or containing minute prisms of zoisite ; small but varying pro- 

 portions of a micro-pegmatite of feldspar and quartz, and a few small 

 patches of quai'tz. In a few specimens the micro-pegmatite has coxToded 

 some of the laths of feldspar. In the diabase from the more central 

 portions of the Little Malali Dyke the feldspar of the micro-pegmatite 

 in places very clearly shows the characteristic cross-hatching of 

 microcline. 



Very pale brown to almost colourless augite is present in ophitic or in 

 places somewhat granular masses, some parts of which show a diallagic 

 structure, with a little more or less idiomorphic enstatite. Here and 

 there the pyroxene masses are bordered with a little green hornblende, 

 small plates of brownish hornblende ai'e of rare occurrence, while in 

 most specimens a very few minute flakes of brown Ijiotite are noticeable. 

 Chlorite is very seldom found in these rocks. Magnetite and titaniferous 

 iron-ore are alw^aj^s present, very irregularly distributed, in varying but 

 usually unimportant quantities. They form irregular grains which in 

 places are boi'dered with leucoxene. Minute prisms of apatite are 

 sparsely scattered through the rocks, whilst a very few extremely 

 minute crystals of zircon are present in a few specimens. 



Where the inner are the more basic parts of the dykes the rock 

 differs but little in structure from the foregoing, except that the ferro- 

 magnesian minerals, and especially the iron-ores, are present in more 



