100 GEOLOGY. 
tubular openings, and to explain the winding channels separating masses of the crust from 
those next adjoining. The sketch (View) was taken a short distance from the point of rocks, 
and represents an outlying part of the spur. All the lower part of the outcrop was covered 
with the orust so as to present rounded surfaces only, but the upper portion was not covered, 
and the black and angular edges of the rocks were distinctly seen. A mass of rock, with a part 
of the calcareous crust broken off, has been introduced into the foreground by the artist, so as to 
convey an idea of the character and thickness of the deposit. 
After clambering over the white crust nearly half-way up to the water-line it was found 
impossible to proceed further with the barometer, and it was therefore set up at the base of the 
rocks. The height of the mercurial column was found to-be 30.248 inches. This was surpris- 
ing, and indicated that we were at or below the level of the sea. The water-line was 
estimated to be about 100 feet above us. 
When wandering over these great masses of rock, and standing in the once sub-aqueous gal- 
leries and passages, with their walls and ceilings of the coral-like crüst, the surfaces looked so 
new and fresh that it was not difficult to imagine that I heard the measured swell of the waves 
resounding in the dim caverns, and it was impossible to resist a feeling of dread that the great 
waters might suddenly return and claim their former sway over the deserted halls. 
From these rocks I obtained a fine view of the Great Desert, stretching off towards the south- 
east in a wide, apparently limitless, plain, its only boundary the unbroken outline upon the 
distant horizon. Not a green spot in all this wide expanse was to be seen ; the bald mountains 
near me were not only free from trees, but there was not even earth to cover the rocks. The 
mountains opposite extended off towards the east until they were lost to sight in a series of low - 
summits of a deep-blue color. The intensity and richness of the colors of the distant hills was 
very striking and beautiful. Blue, purple, and red were the prevailing tints, and their clear- 
ness and depth were remarkable. They were delicately blended one with the other, and pro- 
duced a most beautiful effect, which it is impossible to describe. This variety and intensity of 
color appears to be peculiar to that region, and is probably the result of the extreme purity and 
dryness of the atmosphere, which may be considered as almost free from the vapor of water. It 
is so transparent that small objects can be seen distinctly at extraordinary distances. At the 
time I witnessed the extraordinary and beautiful tints of the distant mountains, the sun was 
going down behind those on our right, and we were soon enveloped in their shadows. As these 
gradually lengthened their outlines were sharply defined on the plain, and their gradual extension 
and progress across the valley to and up the sides of the opposite range could be distinctly noted. 
The shadows were projected on the surface with a peculiar sharpness, the transition from light 
to shade being sudden and distinct. Although the tops of the opposite heights were still gilt 
by the declining sun, there was so little reflection and refraction of the light that we Were 
almost in darkness. 
As soon as the last summit of the opposite range was merged in shadow, a peculiar effect 
upon the air overhead was visible. The whole blue vault seemed traversed by bands of light 
and shade; in fact, the shadows of the mountains were projected upwards into the air, forming 
rays of light and shade, and there was so little diffusion and refraction of the light that they 
were distinctly visible. These shadows, although in reality divergent, appeared to gradually 
converge until they were united in one point far off in tho blue depths of the air, thus afford- 
ing a sublime illustration of the laws of perspective. 
Slope bordering the Mountains.—During the night we travelled over a long slope from the 
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