380 



but on account of the albitic nature of the plagioclase and notable amount of 

 muscovite the rock must be very rich in alkah, and as ferromagnesian minerals 

 are absent, it can be regarded as an Alaskite. 



At the junction of the Mount Gee Creek with the Bolla-Bollana Creek, 

 basalt appears in the section. In this neighbourhood also the strata are cut by a 

 quartz reef of gigantic proportions, continuing out of sight on either hand. 

 Coarse-grained basic rocks, doleritic and gabbroic, outcrop at intervals beyond 

 leading to the Bolla-Bollana Gorge, where the creek cuts through a great quartzite 

 bed. 



Just before reaching the great Bolla-Bollana Gorge quartzite formation, a 

 specially notable area of basaltic igneous rocks was met with. Here some 

 evidence of intrusion was observed, but other parts of the formation were 

 brecciated in a manner which suggested a possible tufifaceous character. The 

 metamorphism which these rocks have undergone obscures their features, and 

 further investigation is needed before any more definite statement can be made. 

 As much of the basalt was noted to be vesicular with amygdular fillings of a 

 white secondary mineral, further support was given to the suggestion of an 

 effusive origin. 



Unfortunately no specimens of these basaltic rocks are preserved for 

 microscopic examination. However, a series of vesicular basic rocks collected 

 not far distant on Paralana Sheep Station by the late W. B. Greenwood, '^''^ and 

 which have very similar features to those at the Bolla-Bollana Gorge, are avail- 

 able, and a description of them follows. It is assumed that they represent other 

 outcrops of this same basic series of lavas elsewhere in the district. 



A moderately coarse-grained amphibolitic rock collected from a large outcrop 

 on the Bolla-Bollana Creek one and a half miles below the junction of the Mount 

 Gee Creek has the following characters as noted in the microscope section : — 



The predominant mineral is actinolitic hornblende in irregular elongated 

 rectangular forms poikiloblastically enclosing minor constituents of the rock. 

 These latter are biotite, muscovite, and calcite as small individuals. There is no 

 definite schistose distribution of the ingredients, the individuals being arranged 

 at all angles to each other. The texture of the rock is typically that of the 

 crystalline schists. This rock, therefore, has arrived at its present condition 

 either by the complete recrystallization of an original basic igneous rock or by 

 the thermal metamorphism of calcareous beds. There is a considerable similarity 

 in microscopic section between this and a certain actinolite rock associated with 

 the copper ore deposit at Yudanamutana. 



Vesicular Lavas from Paralana. 



The vesicular basic rocks collected by Mr. Greenwood are amygdaloidal 

 melaphyres, and comprise five specimens of one class and one of another. 

 The largest member of the first group has the following characters. In the 

 hard specimen it appears as an aphanitic, chocolate-brown rock studded with 

 numerous irregular-shaped, pinkish patches of an amygdaloidal nature. 



The microscope section reveals a basaltic texture modified by metamorphic 

 changes which have almost completely reconstructed the mineral components. 

 Relics of the former plagioclase laths are now somewhat dusty, due to change 

 to fine, practically irresolvable aggregates, the principal constituent of which 

 corresponds closely in character to albite. Apart from the areas actually occu- 

 pied by the relic felspars, the whole section is thickly studded with opaque iron 

 ore, originally magnetite, but partly gone over to limonite. Chlorite is present 



(•5) Mr. W. B. Greenwood was a capable and observant prospector with a good knowledge 

 of the district. He rendered me much assistance on the occasion of my visits. 



