"Latitude 27° 42′ 10′′, longitude 128° 7′ 11′′." It was noon three days after leaving Thirsty Spring, as their last strangely-found well had been designated, and Bob read aloud these observations as he noted them down in his log-book. They had reached the vicinity of Bentley's last camp, and all eyes had been alert for the melancholy symbols of the ill-fated expedition. Away to the east the country extended back in a series of rugged "blows" until they suddenly merged into apparent nothingness; a swelling, white haze obscured the true horizon in this direction, but north and south well-wooded grassy plains stretched into the dim distance. The uttermost edge of the desert seemed to have been reached at last.

"Ay, it was just about here that the camel broke away," said Mackay, musingly, "and over there"—he pointed to the east—"lies the mountain."

"It must be a terrible long way over there, Mac," commented Emu Bill, "for we should see it 'bout forty miles off if it is any size, an' you said it was a whopper."

Mackay looked puzzled; certainly no mountain was visible at this period.

"It must be there," he reiterated grimly.

[Pg 268]

Bob, too, was much exercised over the prolonged absence of the desert sentinel; it had figured so much in all their calculations that it had, indeed, been the initial quest of the expedition.

"There's an extraordinary heat haze rising up straight ahead," said he. "Perhaps it hides that shadowy mountain."

"It's there right enough," said Mackay, again. "I mind well that I didna see it until I was right up against it."

"There's something mighty uncanny about this place," grunted Never Never Dave, who had been gazing around suspiciously.

A gentle zephyr breeze wafted towards them from the obscuring mists, and they sniffed the air wonderingly.

"Blow me tight, boys," muttered Emu Bill, "we has struck old Jimmy Squarefoot's country."

"That are a fact," concurred Never Never Dave, solemnly; "we has come a bit too far on this trip. No wonder poor ole Bentley didn't get back."

"Why, what is wrong?" asked Jack, in some alarm.

"What is wrong?" echoed Never Never. "Why, we must be near Hades, my lad; don't you smell it?"

A strong odour of a pungent, sulphurous nature assuredly filled the air. Mackay was equally mystified with the others, though he did not give expression to his thoughts. He was trying to recall to the minutest incident the happenings of over a year ago in the same district.

"I distinctly saw their tracks," he repeated, half to himself, "and the bones——"

[Pg 269]

"But there ain't no bones now," interrupted Emu Bill. "There's some curious mystery about this here place, there is."

A cry from the Shadow, who had gone exploring on his own account some distance off, drew their attention. It was plain that he had discovered something important, for he semaphored to them excitedly as they looked. Silently they obeyed his summons, and in a few minutes were gazing at the poor relics of the last expedition, where they lay half covered in the sand.

There they were beyond a doubt, a mass of bleaching bones. Reverently they uncovered their heads, then Mackay knelt down by the sad litter, and great, dry sobs shook his breast. His companions turned away with heavy eyes, all but Bob, who remained to comfort the grief-stricken man.

"We may at least bury the remains," he said sadly, "and I think we might put up a small mark over the spot. There are lots of trees about which we could cut down."

Mackay looked at him kindly. "Not yet, Bob, not yet," he muttered hoarsely; "not till I have squared accounts with the wretches who committed this crime. These poor fellows here were murdered after daring the dangers o' the desert; their last mortal remains have awaited my coming here on the surface o' the sweltering sands, and they cry to me for vengeance—and vengeance they shall have before I cover them from the light o' a just Heaven." He rose with forced calm and linked his arm in Bob's. "You shall help me, Bob," he said earnestly; "you of all people have a reason——"

He ceased abruptly as Emu Bill appeared once more.[Pg 270] The tall bushman was apparently much moved, though he strove to hide his sorrow.

"I has just been talkin' to Never Never," he began, in an even voice, "an' we has come to the conclusion that we'll go an' wipe out some o' them skunks who did this. I reckon we'll feel better after it."

Mackay smiled faintly. "I believe we are near the end o' our search for the hidden treasure o' the Never Never," he said quietly. "The invisible mountain must mark the entrance to the land we are seeking, but we may have many a struggle before we triumph, but each difficulty overcome will bring us nearer our goal. Let us move on once more, Bill; I must see the other side o' the mountain——"

"An' nary one o' us is goin' back on you," said Emu Bill, with a grim laugh. "Wherever that there perfume comes from, I reckon Never Never an' me will see the end o' the journey."

They retraced their way to the camels, and in a short time were forcing a trail on into the seething mists. And now the stumbling camel-train experienced great difficulty in negotiating the many dry ravines that lay in their course, and they climbed over the basaltic bluffs which now and again reared their heads above the boulder-strewn expanse, only with the extremest effort. The sun beat down pitilessly on the wayfarers, and here the heat was, indeed, overpowering; it seemed to rise in long, pulsating swells from the bare rocks and hang in a filmy cloud of vapour, through which the eye could see but vaguely, as in a dream-picture. On, on, the pioneers struggled, and as they proceeded, the strange, sulphurous odour became more and more perceptible, until it assailed the nostrils in sharp,[Pg 271] burning breaths. Yet still the vision ahead was clouded by dense white vapours, and the horizon remained obscured. Then suddenly a curious thing happened: the shrouding curtain in the near distance lifted up like a giant screen in a theatre, and through the mists of dispelling ether a dark towering height loomed up vividly.

"The mountain! the mountain!" cried Jack; and truly it was a mountain, and a mountain of so precipitous and forbidding an aspect that it looked like an immense black wall rising into the sky.

"That is just how I ran up against it before," said Mackay, calmly. "It appeared all at once, and I wondered why I didna see it earlier."

"Well, this beats me," growled Emu Bill. "An' why in all the world didn't we see such a colossal monument before? We oughter have sighted that there tower o' Babel at least two days back."

Even as he spoke a great white mask rose from the base of the towering elevation, and in an instant the mighty landmark had vanished from their view.

"I reckon we has had a sight o' ole Jimmy Squarefoot's furnace," remarked Never Never Dave, mysteriously. "An' don't it smell strong?"

"It jest howls," groaned Emu Bill, gasping hard.

Mackay kept an unmoved silence. He apparently had no intention of being surprised at anything; but Bob somewhat eased the minds of the twain by endeavouring to explain the phenomenon they had witnessed.

"The covering haze is nothing but steam," said he. "I think the mountain must be volcanic."

Mackay shook his head. "I shouldna wonder if it is an extinct volcano," he said; "but there was no[Pg 272] lava flow that I remember, and it disappeared just the same."

Notwithstanding the odd happenings during the last few minutes, the camels were not permitted to slacken their pace. Each and all of the party had determined to probe the mystery to the fullest, and the solution was soon forthcoming. As they forced their way into the densest depths of the ghost-like curtain, they became quickly aware of a gurgling, boiling sound almost at their feet. Bob's keen ears were the first to catch the unwonted echoes; but before he could speak a greenish-yellow cloud rolled before his eyes, and he staggered back, choking wildly.

"Ease off, boys," spoke Mackay. "We must scout around an' investigate before we go further."

The whole party, camels and all, were now enveloped in the wreathing smoke-columns, and the sky was hidden from sight. Blindly Bob made a few steps forward, keeping well to the right of the unseen caldron, which now bubbled and foamed spasmodically. The Shadow followed, stumbling and gasping, and within a minute the two found themselves in a clear and untainted atmosphere, and but a yard or so from the base of the gloomy mountain. Loudly they shouted to their companions, and soon the spectral forms of the camels hove into view, with Mackay and Jack treading cautiously at their head. But where were Emu Bill and Never Never Dave? They seemed to have vanished completely.

"They were alongside Jack an' me a minute ago," said Mackay, gazing wonderingly around.

"I reckon I'll go back an' see if they've stopped to look at the scenery," grunted the Shadow; and he made a dash into the heavy fumes once more.

[Pg 273]

The three who waited by the camels heard a startled cry, followed by a faint splash, then all was silent. Hastily Mackay seized a camel pack-rope, and would have rushed off after him, but Bob interfered.

"I think I can guess where to find them," he said. "Let me go."

With reluctance Mackay saw him depart; but before Bob had entered the chaos the swelling mass rose before him, disclosing in his track a broad, pit-like cavity. Hurriedly he strode to the edge of the caldron; but ere he reached it the Shadow climbed out of its seething depths wet and dripping, and saying strange things to himself. Immediately behind him Never Never Dave's head popped up, and an eloquent flow of language was let loose upon the air. Lastly, Emu Bill scrambled into the open. He looked savagely around for a moment, until he caught sight of the Shadow, and his wrath overflowed in a torrent of abuse. All three were bedraggled enough looking specimens; but the last arrivals were considerably worse off than the Shadow in that respect—their hair was covered with a greenish scum, which spread down over their faces and almost blinded them.

"It was all that wretched young Shadow's fault!" roared Emu Bill.

"But you were there first, Bill," remonstrated Mackay, laughingly.

"Of course we were. We went plump into the filthy boilers; but we got a good grip o' the sides, and were sliding out quick an' lively, when, blow me! if that howlin' scarecrow didn't bounce down on top o' us, an' sent us swimmin' like tadpoles to the bottom. Ugh!"

But their indignation quelled speedily when they learnt[Pg 274] how excellent had been the intentions of the much-maligned youth.

"It must be a hot spring," said Bob.

"And there are more of them," cried Jack. "See, they are scattered all round the foot of the mountain."

"I reckon it is hot, right enough," grumbled Emu Bill. "I'm just 'bout turned into a Salamander, I is."

As Jack had noticed, quite a number of similar indentations formed a line right along the base of the mountain, and in each yawning crater examined, a greenish-yellow fluid bubbled tempestuously. High overhead the smoke-wreaths dissembled into thin air, and for a brief space all was beautifully clear. Then a dull rumble like the mutterings of subdued thunder was heard, and immediately snowy puffs of smoke issued from the strange cavities. The denser fumes rapidly spread along the ground like a turbulent, foaming sea; then the whole seemed gradually to rise upwards and suspend as a filmy pall before the face of the mountain. Yet, strangely enough, the noxious odours were now almost absent.

"Ay, it's vera marvellous," said Mackay, with a sigh. "So does Nature protect her treasure-houses."

"It's a wonder you managed to get through without accident when you came," Bob observed thoughtfully. "But, then, it's possible the line was clear when you passed."

"A breeze of wind would have lifted that fog," hazarded Jack. "I should think that on some days the clouds would not be nearly so constant nor so thick."

"You're possibly right, Jack," mused Mackay, looking upwards. "See how the smoke curls in to the west now."

They all followed his gaze, and, surely enough, the mists[Pg 275] appeared to bend over before a powerful air current and break off into numberless flying patches of lambent spray. Assuredly, a fairly strong blast must be blowing on the mountain summit, though all was serene and unmoved below. They now bethought themselves of having an inspection of the wonderful elevation, which they had reached after so much weary striving. There it stood, gaunt and bare, precipitous in outline, and rising almost sheer to a height of over eight hundred feet, and as far as the eye could reach in either direction along the base, the same grim barrier appeared, but it curved in almost imperceptibly at each limit of observation.

"The monument might stretch across into Queensland," said Emu Bill, "if we tried to follow it round. I vote we does a scramble over the top."

"I tried that before, Bill," answered Mackay, "but I'm going to try it again. Only we'll look for the easiest side before we start."

"But what about the camels?" asked Jack. "We can never get them over it."

"An' we've got to remember that there's an all-fired quantity o' bloodthirsty niggers about," said Never Never Dave.

"Suppose we hobble them out on the other side of the smoke," suggested Bob. "If the blacks stay beyond the mountain they couldn't very well see them so close in, and the camels are too tired to wander much."

"It's a risk, Bob," said Mackay; "but the whole journey has been a risk, an' it's the best we can do. We can keep an eye on them from the top—if we get there."

"An' our rifles can speak for us from there well[Pg 276] ............