THE DESERT SANDSTONE. 307 



According to all appearances this mineral easily decomposes 

 through atmospheric influence, assuming at first chatoyant colours, 

 then turning to reddish-brown, and ultimately, beneath a thin 

 coating of hydrous oxide of iron, changing to a brownish-red mica 

 ('Rubellane')." 



Prof. Liversidge (op. cit. p. 117) gives many localities for the 

 occurrence of olivine, besides many magnesian products which may 

 be supposed to have been derived from the decomposition of 

 chrysolite in basalt ; but it is not necessary to cite the passage. 

 It may be mentioned however that at the railway cutting along 

 the Main Range, about 100 miles west of Brisbane, both tunnels 

 and cuttings are made through ash deposits derived from a large 

 extinct volcano on the edge of the Darling Downs. Over the 

 ash-bed there is a distinct overflow of basalt which is conspicuously 

 full of olivine, the masses being sometimes of large size. The 

 section is very instructive, for the ash-beds are partly decomposed 

 and in some respects remind one of the Nepean Sandstones near 

 Sydney, New South Wales. At the junction of the lava stream, 

 the ash-beds are conspicuously discoloured from the action of the 

 heated basalt, forming long lines of red, pink, and other colours, 

 like the effect of burning in a kiln. 



Though the ancient character of the ash-beds of North Australia 

 may be inferred from their chemical metamorphism, yet they are 

 the newest deposits that are to be found in this region. They lie 

 on the top of all other formations which they cover, as already 

 stated, to a varied depth. The following description of some of 

 the beds exposed is taken from different portions of my report. 



McMinn's Bluff. — The road from Pine Creek by the side of the 

 telegraph line passes along a valley formed by a flat sandstone 

 table-land on the west side and a low slate range on the east side. 

 The table-land forming the western boundary of the valley is at 

 its southern end a long narrow range, covered with a stratum of 

 stone, which stands out like a rampart some 30 or 40 feet thick, 

 giving a castellated appearance to the flat-topped hills. As 

 the range is followed north it is broken into three or four small 

 outliers of white and red colours. They look like ramparts and 

 fortresses, and are of very picturesque appearance. They all have 

 a steep incline for about two-thirds of their height, and then become 

 rugged for some distance, and then suddenly precipitous for 30 or 

 40 feet to their flat-topped summits. One of these hills is of fiery 

 red on the top, and it is joined by a low saddle to another outlier, 

 which is capped with picturesque cliffs which are white. 



The section of these hills is as follows : — Granite, 90 feet at 

 least, it may be more, but the line of junction is concealed by 

 weathered masses of rock, which have fallen down from the cliffs. 

 Then follows 100 to 150 feet of coarse red sandstone. Then 30 



