Doc Kates sat down. ‘Well,’ he said, ‘what brings you back to the old drugstore? You didn’t just come here to chew the fat, did you?’

Collis shook his head. ‘It’s the tunnel we’re digging through to the summit. We made very slow headway during the winter, because all we had was black powder. Then Leland McCormick brought us some nitroglycerin. Blasting oil, they call it, and that cuts through the mountain like cheese.’

Doc Kates nodded. ‘I see your problem. They’ve banned it now, haven’t they, after that big bang in San Francisco? So how are you going to blow your way through to the pass?’

Collis sat back in his chair. Through the window, he could see Doc Kates’s vegetable garden growing, just as it had last year and the year before, and always would do, until he died. ‘As far as I understand it, nitroglycerin’s made out of sulphuric acid, and nitric acid, and glycerine.’

‘That’s right,’ said Doc Kates, looking at Collis warily.

Collis looked right back at him. ‘Supposing someone was to bring these three ingredients — which aren’t illegal when they’re brought into the state of California separately — supposing someone was to bring them up the mountain, just as they are, and then supposing someone was to mix them together right up by the tunnel we’re blasting out and give us a ready supply of the stuff?’

Doc Kates took off his spectacles. His eyes were watery and pale, faded by years of Sierra sun and years of Sierra snow.

‘You’re asking me to do it?’ he wanted to know.

‘Only if you’re willing.’

‘It’s pretty volatile stuff. One mistake and you’re a damned sight higher in the air than Summit Pass.’

‘You’re a careful man.’

‘Yes, I guess you could say that I am.’

‘Then you’ll do it?’

‘Let’s have ourselves a cup of coffee first,’ Doc Kates said. ‘Do you fancy a ginger cookie? I’ve a fresh-baked batch in from Mrs Elias.’

Collis stood up, pushing back his chair. ‘I don’t have much time, Doc. That tunnel has to be bored, and bored quick.’

Doc Kates frowned at him. ‘All right,’ he said, slowly bringing the cups to the table and setting them down. ‘All right, I’ll do it. Do you have time for a cup of coffee now?’

All during the long summer of 1867, the tree-lined mountains below Summit Pass shook and echoed with the sound of explosions. Inside the solid granite tunnel, it was stifling and dusty and intolerably hot, but the Chinese blasters drilled incessantly into the rock face, laid their charges of freshly-brewed nitroglycerin, and gradually blew the inside of the mountains out.