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THE AUSTRALIAN NATURALIST. 119 
stream. Once eon the bend we find ourselves in a valley with 
one sloping bank of uniform steep grade stretching to our right, 
covered with swamp-loving grasses and a few sw amp trees. The 
other bank of the valley is a low cliff, the wall of a bench, 
similar to the one described earlier, but only a few feet in height 
measured from stream level. The place is worth careful map- 
ping, in order to discover the cause for the change in scenery 
that occurs there. I should be glad if some of our members 
could supplement these notes by some constructive work of this 
nature; whilst I look forward to the time when we can publish 
a series of photographs, or, better still, stereo- photographs, 
showing to many geologists in other parts of the world our trea- 
sures, whether of scenery or of geology. 
The neighbourhood of Wall’s Cave is perhaps the most in- 
teresting of the whole Canyon. Here a volcanic dyke four 
feet in thickness penetrates the sandstone rock, and is more 
easily weathered than the sandstone. Standing on a “bench” 
overlooking Wall’s Cave, this dyke may be traced for some dis- 
tance cutting abruptly across the Canyon, which meanders in a 
big loop. The creek can be traced from its sourees in the large 
swamp below Medlow, down a long valley with grassy banks, 
into this short section of the Canyon. Halfway round the 
big loop the river disappears: the remainder of the loop being 
a dry valley. Descending to the river we find that the water 
flows through a tunnel in the solid rock, at the very place where 
the dyke penetrates the sandstone. Evidently the copiousiy 
jointed dyke rock has been worn away by stream action, and 
the tunnel so formed, widened by the erosion of the sandstone 
_in the immediate neighbourhood of the dyke, so that the bow 
of the canyon bend is now a dry valley, although its level is 
little above the level of the tunnel. We are here face to face 
with a recent structure in the geological sense, but the word 
recent must not be interpreted too literally. The Blue Moun- 
tains and their gullies are in many ways well preserved fossils. 
Denudation has proceeded so far that the catchment areas of 
the present streams are only a small fraction of the areas drained 
when the Grand Canyon and its co-eval gullies were being ac- 
tively carved out of the solid sandstone; and the cutting power 
of the. streams has been greatly diminished in consequence of 
the reduced flow of water. 
The effect of the Blue Mountain scenery on different minds 
varies in a peculiar way; but the writer has experienced the 
