Titus Livius (Livy), The History of Rome, Book 21
Rev. Canon Roberts, Ed.

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At last, when men and beasts alike were worn out by their fruitless exertions, a camp was formed on the summit, after the place had been cleared with immense difficulty owing to the quantity of snow that had to be removed. [2] The next thing was to level the rock through which alone a road was practicable. The soldiers were told off to cut through it. They built up against it an enormous pile of tall trees which they had felled and lopped, and when the wind was strong enough to blow up the fire they set light to the pile. When the rock was red hot they poured vinegar upon it to disintegrate it. [3] After thus treating it by fire they opened a way through it with their tools, and eased the steep slope by winding tracks of moderate gradient, so that not only the baggage animals but even the elephants could be led down. [4] Four days were spent over the rock, and the animals were almost starved to death, for the heights are mostly bare of vegetation and what herbage there is is buried beneath the snow. [5] In the lower levels there were sunny valleys and streams flowing through woods, and spots more deserving of human inhabitants. [6] Here the beasts were turned loose to graze, and the troops, worn out with their engineering, were allowed to rest. In three days more they reached the open plains and found a pleasanter country and pleasanter people living in it