These two authorities give the only notices of the pres- 

 ence of pleonaste in grauophyric or vermicular form, which 

 I have been able to And, and neither of them describes rocks 

 as simple in character as those of Duudas. We have, there- 

 fore, close to Sydney, a unique opportunity for the study of 

 the origin of spinel in igne< ms rocks, ami its physic,. -chemical 

 characteristics. It is to be hoped, in view of such a work 

 being undertaken here, that any one finding a spinel gabbro 

 will present a portion of the specimen to one of the museums 

 of this city, so that it may become available for future use. 

 Brief Descriptions of Particular Specimens. 



a. Spinel both in felspar and pyroxene though preiVninc; 

 latter. Possibly this is due to the earlier crystallising 

 pyroxene using the spinels for nuclei leaving a fairly clear 

 field for the felspar. This latter is bytownite-labraborite. 



c. Rock Jias a slightly banded arrangement of pyroxenes. 

 They are just commencing to decompose peripherally. The 

 felspar is labradorite. This may be classed as a hypers- 

 thene gabbro. 



d. A very fresh rock with labradorite as the felspar. 

 Rhombic pyroxene is slightly in excess of the monoclinic, 

 thus causing the rock to pass over into the group of norites. 

 Diallage structure is partially developed in the pale pinkish- 

 brown augite. The rhombic pyroxene, without schiller 

 structure," is practically an enstatite. (See Fig t 10.) 



e. The predominant mineral is a colourless diopside with 

 very slight trace of a diallagic parting. The development 

 of the structure described on a previous page as probably 

 due to an intergrowth of rhombic pyroxene is present to a 



f. Felspar a basic labradorite ; pyroxene a pale diopside 

 only slightly diallagic, decomposing into chloritic material. 

 The spinel is sometimes irregularly placed in the crystals, 

 sometimes running perpendicularly to the boundary. 



