HORN EXPEDITION — PETROLOGY. 91 



pit'ces (if ;i, liaik- biowii iiiie;i. are- present as alteration products. This rock 

 occuis iiitiiulini;- gneiss between ISlip-panel Uap and EUery Creek. 



Epidote Rock. No. 65. Plate II., Fig. 4. Sp. gr. 3-20. 



Tiiis- rock consists of epidote, hornblende, and a little felspar mostly .secontlary. 

 Tile epidote, which forms more then one-half of the rock, is irregularly granular, 

 with the usual jileochroism, and is probably due to the alteration of the felspar, 

 and in .some degree of the hornblende. The latter occurs in crystals, which appear 

 to have suii'ered considerably during the formation of the epidote. The pleochroism 

 of hornblende appears bluish-green pai-allel to c", yelhjwisli-green parallel to /', and 

 pale yellow jiarallel to a. The pleochroism is unconnnon, and a reedy tibrous 

 structure is apiparent in most of the sections, Plate II., Fig. 4. In pai'ts it shows 

 alteration into flakes of a br(jwnish mica. This rock occurs on the side of dolerite 

 intrusion between Slip-panel Gaii and Ellery Creek. 



Amphibolite. No. 16. 



This rock is somewhat foliated with a little secondary (piartz in veins. It 

 is almost entirely composed of crystals of hoi'nblende having a remarkable 

 pleochroism. The e.vtinction is 20' on tlie clinopinacoid. The pleochroism is t 

 strong greenish-blue, Ij olive green, and 3 ])ale yellow. 8ome sections exhibit a 

 very finely striated appearance. A certain amount of mica is also present, some of 

 which is a dark biotite, the greater portion, however, is a bluish grec^n Inotite, 

 which is practically uniaxial, and has a pleochroism varying from <lark gr(;en to 

 yellow. This rock comes from Claraville in the East McDonnell Ranges. 



Uralitic amphibolite. Nos. 73, lO-"), 123. Plate I., Fig. G. Sp. gr. 3()G. 



In the mass it is compo.sed of greyish-green tibrous hornblende, with a certain 

 amount of a paler green augite scattered through it. In section by far the grt'ater 

 portion of the rock is seen to be composed of hiirnblende, the pleochrosim of which 

 is pale bluish-green, yellowish-green to nearly colourless. The crystals are 

 irregularly interwoven without definite boundaries, and have a coarsely fibrous to 

 patchy appearance in polarised light. The most striking feature is the fre(pient 

 development of the prismatic cleavage in the hornblende. Amongst the horn- 

 blende irregular patches of colourless augite grains remain, and there is little doul^t 

 tliat these are the remains of pyroxenic minerals, which have yielded the horn- 

 blende l>y paramorpliic change. The hornblende is in fact fairly typical uralite. 

 This rock comes from Plack Knob, near the head of Ellery Creek, in the 

 McDonnell iianges. 



° 12A 



