547 
tlie similarity of the com2)onents of these sandstones to those of the Mandarana lavas, 
and their ajiparent unconformability to the underlying rocks, they are regarded as being 
subaqueous trachytic tuffs of the same age. 
“ Another fragment of rock of this class is seen eapjjing the Pinlayson Hills, 
twenty-two miles north-west of Mackay, and about a mile and a-half from the sea- 
coast, where it forms a sugarloaf-shapod pieak, resting on granite, and rising to a height 
of about one hundred feet above the summit of the hills. (Plate 46, fig. 2.) This rock 
has a matrix of a purple-grey hue, with well-marked banded or ribboned structure, in 
which the lines of flows can often be seen to bend round the larger sanidine crystals 
embedded in the base. As a whole, the rock is much more compact than the Mandarana 
trachyte, and, like it, forms rudely liexagonal columns. 
“ On the eastern bank of the Yictor Creek, in Selection 590 (Parish of Ossa), a 
well-marked escarpment of trachyte, forming McKenzie Crags, occurs. Here the lava 
rests upon the volcanic series.* Several other isolated fragments occur near liighwater 
mark at Pocky Bluff, about four and a-half miles south of Cape Hillsborough. 
Near the head of Niddoe’s Creek, one of the watercourses draining the western side 
of that range of hills lying between the main range and the coast, a trachyte lava of 
a somewhat different character is seen, dijjping south-east at an angle of 12°, and 
resting upon the sedimentary rocks of which this range is made uji. Lithologically, 
the rock may be called a quartz-trachyte, and throughout it presents a great uniformity 
in its physical characters ; it is made up of a light-grey porous matrix, in which 
quartz, sanidine, and small specks of what aj)pear to be hornblende are embedded. 
“ In the Parish of 8t. Helens, on the south bank of Alligator Creek, a lofty range 
of mountains, the ‘ Pinnacles,’ which form a ‘ corry,’ encircling one of the branches of 
this creek, a great thickness of lava occurs. The rocks are trachytes of a brownish-grey 
colour, and with which fine grained trachyte tuffs are associated. The lava sheets have 
their steeper faces southwards, and appear to dip in a general northerly direction. 
“ One of the sources from which some of these lavas and ashes have been ejected 
appears to be Mount Barren, a steep, trijjle-peaked mountain, the highest summit of 
which is about 2,000 feet above sea-level, and which is almost surrounded by the head 
waters of St. Helen’s Creek. The rock of which this mass is made up is greyish-white 
in colour, and somewhat porous, with a mean specific gravity of 2‘.56. In tlie matrix 
small crystals of sanidine and minute specks of a black mineral — probably hornblende 
■ — can be recognised ; throughout the whole of the mass the rock retains very much the 
same character. The mountain rises perpendicularly from the alluvial flat on the north 
bank of St. Helen’s Creek, and in Barren Creek the mass is seen to be intrusive 
through an ‘ orthoclase porphyry,’ upon which the sedimentary strata [of the Bowen 
Kiver Series] are seen to rest. No p)ercej3tible amount of alteration was detected in 
any of the sections in which its intrusive character was observed. 
“ Another denuded wreck of an old volcano is to be found in Mount .Tukes, some 
1,800 feet above sea-level, and situated on the bank of Neilson’s Creek, and abou 
twenty miles distant from Mackay in a north-westerly direction. The mean sp)ocific 
gravity of the rock, from specimens in different parts of the mountain, was found to be 
2'55. Different jjarts of the mass pi’esent different characters, but generally two 
vai-ieties can be recognised : — 
(а) A coarse-grained rock in which a matrix can scarcely be said to exist, and 
(б) A second in which crystals of sanidine and plagioelase are embedded in a 
micro-crystalline base, which, with the aid of a lens, is seen to be made up 
of small crystals and crystalline grains of sanidine and hornblende (?). 
* At the base of the Bowen River or rermo-Carboniferous Formation, 
