84 HORN EXPEDITION — PETROLOGY. 



FELSPARS. 



Plagioclase. No. 147. Plate III., Fig. 2. 



This section exhibits what apj^ears to be an inter-growth of felspar and mica. 

 There are several pieces of mica with well-marked basal cleavage, showing that the 

 section is cut at right angles to the basal plane. All the pieces of mica are in 

 optical continuity, as shown by their extinguishing simultaneously. The felspar 

 is a plagioclase with very low extinction angle. The section was obtained 

 by grinding down a cleavage flake parallel to the brachypinacoid (010). On 

 examination "under crossed nicols " the felspar and mica are seen to extinguish 

 practically sinudtaneously. It seems probable that the several pieces of mica 

 formed part of a single plate, which lay parallel to tlie basal plane of the felspar. 



Microcline. No. 18^. Plate III., Fig. 4. 



This is a section of microcline obtained by grinding down a ba.sal cleavage 

 flake, and shows in an excellent manner the " cross-hatched " structure. In the 

 microcline are to been seen strings of a plagioclase, with low extinction angle, 

 arranged in a roughly parallel manner. The several plagioclase strings are in 

 perfect optical continuity with each other, and exhibit the albite type of twinning. 

 They probably occupy the sites of cracks, whicli have been filled by a secondary 

 growth of felspar diff'ering in composition and micro-structure from the original 

 felspar. 



Plagioclase. No. 15(^. Plate III., Figs. 5 and 6. 



This is a plagioclase that is very common in the.se granites of the Hart 

 Range. The albite type of twinning is developed in a wonderfully perfect manner, 

 and the angle of extinction measured from these twinning planes Hoea not exceed 

 2" or 3°. The sections were ol)tained by grinding down cleavage flakes parallel to 

 the basal plane (001). 



In Fig. 6 is given a drawing of another basal section, in which masses of 

 felspar, exhibiting the characteristic microcline structure, are seen to be enclosed 

 in the plagioclase. Whether these represent original inter-growths, or cavities 

 sub-sequently fllled with felspar of a diflerent composition to the original, or a 

 product derived from the alteration of plagioclase in s/fu, it is diflicult to decide. 

 The last seems the most probable explanation. 



