Why does the sleeper in the tent dream that an elephant is sitting on his chest?What does he think has happened?

From the text:

There you dream that an elephant has suddenly sat down on your chest, and that the volcano has exploded and thrown you down to the bottom of the sea—the elephant still sleeping peacefully on your bosom. You wake up and grasp the idea that something terrible really has happened. Your first impression is that the end of the world has come; and then you think that this cannot be, and that it is thieves and murderers, or else fire, and this opinion you express in the usual method. No help comes, however, and all you know is that thousands of people are kicking you, and you are being smothered.

Somebody else seems in trouble, too. You can hear his faint cries coming from underneath your bed. Determining, at all events, to sell your life dearly, you struggle frantically, hitting out right and left with arms and legs, and yelling lustily the while, and at last something gives way, and you find your head in the fresh air. Two feet off, you dimly observe a half-dressed ruffian, waiting to kill you, and you are preparing for a life-and-death struggle with him, when it begins to dawn upon you that it’s Jim.

“Oh, it’s you, is it?” he says, recognising you at the same moment.

“Yes,” you answer, rubbing your eyes; “what’s happened?”

“Bally tent’s blown down, I think,” he says. “Where’s Bill?”