44 HORN EXPEDITION — GENERAL GEOLOGY. 



ably exhibits the characteristic " cross-liatching " of niicrocline, and a plagioclase 

 belonging to the soda end of the soda-line series, with very low extinction angles 

 and very fine polysynthetic twinning. The mica is also of two kinds, a black 

 variety biotite, and the potash variety muscovite. Beryl and tourmaline occur as 

 accessory minerals. 



The granitiie from an intrusive dyke on the opposite (western) slope of the 

 mountain spur to that on which the Oolgarna Claim is situated has an almost 

 identical niineralogical composition to the Oolgarna rock. There are the same two 

 varieties of felsjiar, a niicrocline and a plagioclase, with well-developed polysynthetic 

 twinning and small extinction angles. Of the micas, muscovite alone appears to be 

 developed in this dyke, and hugs in a marked manner its southern wall. Here 

 also tourmaline and beryl occur as accessory minerals, with the addition of some 

 small red garnets. A crystal of beryl was obtained from this rock, which measures 

 three inches in diameter and six inches in lengtli. Its form is that of a perfect 

 hexagonal prism, the terminal ends of which are not present. It exhibits macros- 

 copic zoning, the several zones of the mineral having a slightly different tint, with 

 a central nucleus of quartz and felspar. Its colour varies from pale green to 

 yellowish green. The plates parallel to the basal plane, obtained by taking advan- 

 tage of the somewhat imperfect basal cleavage, were found to yield in convergent 

 polarised light excellent interference figures having a negative sign. Some of the 

 hand specimens from this dyke consist entirely, or nearly so, of felspar, sometimes 

 microcline, at other times a soda plagioclase ; others consist entirely of quartz and 

 muscovite, while yet others again consist entirely of mica. The order of crystalli- 

 sation, as determined from the hand specimens, is mica, quartz, and felspar. The 

 felspar has crystallised out last, as it frequently encloses entire crystals of quartz 

 and mica. The mica has crystallised out first, for where the quartz comes into 

 direct contact with a crystal of mica its (the quartz) outline is found to have been 

 determined by that of the mica. 



A specimen (No. 93) which was obtained in the McDonnell Ranges, near the 

 head of EUery's Oreek, represents a type of gneissic granite, which is not uncommon 

 in these ranges. There can be no doubt that it is a granite which has undergone 

 a certain amount of crushing, but not enough to make its origin a matter of doubt. 

 It consists for the most part of microcline and (juartz, which are often inter- 

 crystallised in a manner similar to that observed in graphic granite. The rock in 

 the hand specimen does not appear to be foliated, whereas, in the field, a coarse- 

 bandeil foliation was observed. Masses of intercrystallised microcline and quartz 

 are separated by thin irregular laminte of biotite flakes. 



