590 
easily accessible. But an immense area of tlie basalt stretches from the Mulgrave to the 
heads of the Johnstone and westwards to near Herberton, and under this the auriferous 
drifts jirobably remain intact. 
It seems probable that the Yalleys of the Mulgrave, Russell, and Johnstone 
were dammed up by a volcanic outbreah, so as to form a vast lake, iu which for a time 
a fine siliceous sand was quietly deposited ; that the barrier was at length broken down 
by the stream at the outlet of the lake, and numerous torrents removed the greater 
part of the sand and brought down gravel charged with gold ; that a second barrier 
was thrown up and a great thickness of fine silt again deposited ; and that over the 
nearly level surface thus produced immense flows of basalt were finally poured out, 
filling up the lake. The waters draining this area, if the lake had not been filled up, 
would have found their way to the sea by a single outlet, instead of forming as they do 
three independent rivers. The immense amount of denudation since the outpouring of 
the basalt is evident when we consider the number and depth of the valleys, which have 
been carved out, in places, to a depth of more than a thousand feet, through basalts, older 
river and lake beds, and slate and granite rocks; but the lesson will be impressed more 
deeply than any description could do on anyone who can stand on a neighbouring 
eminence and try to reconstruct, in his mind’s eye, the shores which contained the lake.* 
Gold has for some time been worked, chiefly by Chinese, in the Recent alluvia 
of the Russell and Johnstone. This gold, which is mixed with stream tin, in all 
probability came, for the most part, from the “ leads” under the now denuded basalt. 
Probably only a small portion of the alluvial gold came directly from the reefs exposed 
in the slate and granite country since the denudation of the basalt, as a comparatively 
small area of this country is yet exposed, and not many reefs are known in it. 
I am informed by Mr. John Falconer that on the dividing range between the 
Dawson and Condamine, lava-form beds of basalt overlie the Desert Sandstone in 
places. 
I have also been informed that a cake of basalt overlies the fragment of the 
Desert Sandstone left on the crown of the ridge dividing the head waters of the Langlo 
River from those of the Coorni Paroo, between Tambo and Adavale. 
It may be thought remarkable that numerous sheets of lava should have been 
poured out, one after another, and retain such an even surface. IMo doubt the fact that 
this has been the case argues a high degree of fluidity in the lava. Some remarks by 
Captain Clarence E. Dutton, of the United States Army,t on the Volcanic Rocks of 
the Zuni Plateau, New Mexico, may be quoted on (his point ; — 
“ It is plain that the lava sheet which forms the capping of the mesa was 
gradually accumulated by the repeated outpourings of numberless local vents thickly 
scattered over its broad surface. And yet it may seem a little strange at first that this 
diffuse form of volcanic action should have produced a surface which is so little 
diversified. Judging by the analogy of other regions we should have expected to find 
the thickness of the lava-cap extremely irregular and studded with large cones with 
intervening valleys of considerable depth. In reality the thickness, as we recede from 
the base of Mount Taylor, diminishes very slowly, and even at the extreme northern 
end is quite three hundred feet. In truth Mount Taylor docs not appear to have con- 
tributed much, if any, to the lava which is more than five or six miles from the 
immediate base of its cone. The surface of the mesa away from the mountain is but 
little diversified, and the local cones are for the most part insignificant features. All 
* Report by R.L. J. on the Geology of the Russell River. Brisbane ; by Authority : 1888. 
t “Mount Taylor and the Zufii Plateau” in Sixth Ann. Report V. S, Oeol. Survey, 1884-85. 
