He looked up. 'Time you were awake. Sure, I've seen it -one of the Tassili frescoes.'

I turned back to stare at it. The colours seemed as fresh as though it had been painted the week before and there was a fluency and elegance of line in the drawing of the cattle which any modern painter would envy. 'How old is this?'

Byrne came into the cave. 'The cattle? Three thousand years, could be four.' He moved along the wall of the cave until he came to the end. 'This is older – this mouflon.' I scrambled to my feet and joined him. The wild sheep was more crudely executed. 'Eight thousand years,' said Byrne. 'Maybe more, I wouldn't know.'

I began to examine the wall more carefully, looking for more treasures, but he said brusquely, 'No time for that We've a long way to go. Wake Billson.'

Reluctantly I turned away, woke Paul, and then helped to make our breakfast. Not more than half an hour after I had woken we were on our way again, threading the canyons of the Tassili. An hour later I saw the green of trees, big ones lofting more than fifty feet. The branches were wide spread but twisted and gnarled. I said, 'There must be water here,' and pointed. 'Cypress,' said Byrne. 'Those can have a tap root a hundred feet long and going straight down. And they're older than Methuselah; maybe they were here at the time the guy was painting those cattle back there in the cave.'

We left the trees behind and marched in silence and again all was silence except for the clatter of stones and the snorting of the donkeys and an occasional word passing between Atitel and Hami. There wasn't much to say about what we were looking for – everything had been said to exhaustion. And there wasn't much to say about Lash, either. If he was coming up behind he'd either catch us or he wouldn't.

We stopped briefly at midday to eat, and again at sunset, and then pressed on into the moonlit night I thought it unsafe and said so, but Byrne was confident that Atitel knew what he was doing, more confident than I. Again we stopped at about nine and I found another cave. To my surprise I was not as tired as I had expected to be, and Paul was better, too. I looked at him as he unslung a jerrican from a donkey and thought of what Isaacson, back in Luton, had called him. A nebbish! The total nonentity.

It was true! Hours had gone by at a time when, even in Paul's presence, I had not given him a thought. When we drove in the Toyota he always sat in the back and wasn't under my eye. On this, and other, desert marches he always brought up the rear. He said little, never commenting on what he saw, however wondrous, but just stubbornly put one foot in front of the other. And he never complained, no matter how he felt. It was something to say for Paul but, all the same, he might just as well not have been there. The nebbish!

As for Luton – that was a million miles away, on another planet.

We fed on dates and dried mutton and I asked Byrne what progress we were making. He chewed vigorously, then swallowed. 'Not too bad. Atitel reckons on less than a day and a half. He says he'll see a landmark he knows before dark tomorrow.'

'What about Lash?' I said. 'And Kissack?'

'What about them? At Tamrit we left them at least eight hours behind, and you can add another three hours tonight because they won't be moving at night. I guess we're a full day ahead. And they don't know where we're going.'

'We've been leaving tracks. I've noticed. Prints in the sand and donkey droppings.'

He nodded. 'Sure. But we've also been moving a lot on rock and leaving no trail. They can follow us if they know how but it'll take up a lot of time, casting around and all. That puts us another day ahead, maybe two.' He took another bite of mutton and said casually, 'We might run into them on the way back.'

That's nice.'

He grinned. 'I'll ask Atitel to take us back another way.'

When I awoke the next morning I eagerly scanned the wall of the cave but, to my disappointment, it was bare rock. Kami had baked bread in the hot sand under a fire, and it was crunchy in the crust and very tasty if you ignored the gritty sand. After breakfast we set off again, Atitel leading the way through the shattered wastes of the Tassili n' Ajjer.

The worst thing that could possibly have happened occurred at mid-afternoon, We were picking our way through a particularly bad patch where, for some reason or other, the wind action on the sandstone columns had been accentuated. The grinding action of sand-laden wind against the bases of the columns had felled a lot of them and, in their fall they had smashed and broken, leaving a chaos of debris through which it was difficult to negotiate our way.

Suddenly the donkey which Atitel was leading brayed vigorously and plunged, butting him in the back so that he fell. He gave a cry and Byrne ran up and stamped at something on the ground. When I got to him I saw it was a snake.

'Horned viper,' said Byrne, and ground its head to pulp under his heel. 'It scared the donkey.'

It had done more than that because Atitel was sitting up holding his leg and groaning. Byrne examined it and looked up at me. 'It's broken,' he said flatly. 'Christ!' I said. 'What do we do now?'

'Make a splint for a start'

That wasn't as easy as it sounded because we had nothing suitable for a splint other than the barrel of the rifle. Unexpectedly, it was Paul who came up with a good idea. He tapped a jerrican which was hanging on the flank of a donkey and it rang hollowly. 'This empty?'

'Yeah.'

'We can bash it with rocks,' said Paul. 'Flatten it. We ought to be able to make some sort of rigid splint.'

'We can do better than rocks,' said Byrne, and went to a donkey and unpacked the cloth-covered bundle he had brought. From it he produced a hammer and a cold chisel. 'Get that can on the ground.'

It took time and the desert rang with the sound as it echoed from column to column but eventually we splinted Atitel's leg, padding it first and then binding the metal with strips ripped from a gandoura. He had stopped groaning and looked on interestedly as we did it.

When we had finished Byrne squatted next to him an d uttered the first words of what proved to be a long conversation. I said to Paul, 'God knows what we'll do now. From what Byrne told me last night we're ten or twelve kilometres from where the old man said he saw the plane.'

'We'll go on.' Paul's face was set in stubbornness. 'Be reasonable.' I waved my hand at the chaos all about us. 'How the hell can we find it without a guide? This, Paul – this bloody Godforsaken land – is the reason it wasn't found in the first place. You could walk within ten yards and never see it.'

'We'll go on,' he said. 'And we'll find it.' I shook my head and looked to where Atitel was drawing with his finger in the sand. Byrne was asking questions. I shrugged and went to help Hami adjust the harness on one of the donkeys where the edge of a jerrican had chafed and worn a sore spot in its hide.

Half an hour later Byrne stood up. 'Okay; Atitel and Hami are going back. The old man can ride a donkey and Hami will lead another with enough food and water for the two of them. He'll take Atitel to Tamrit and then go down into Djanet for help.'

I said. 'They might run into Lash.'

'I've told them about Lash. They know enough to keep clear of him. Hami will go back a different way.' He laughed shortly. 'I said it's a blood feud; they understand that.'

'And us?'

'We go on.' I looked at Paul, who was grinning. 'Atitel's landmark is unmistakable, according to him. It's a big rock column about two hundred feet high and split from the top to half-way down as though someone has driven a wedge into it – you know, like splitting timber. He says all we have to do is to keep going the way we are now and we should see it in a couple of hours.'