Mary Shelley refers to the account of the long Greek retreat from Armenia in Xenophon's
Anabasis, 4.7, quoted here in the translation of Carleton L. Brownson (Cambridge:
Harvard University Press, 1918):

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[18] Leaving this land [of the Chalybians], the Greeks arrived at the Harpasus river,
which was four plethra in width. From there they marched through the territory of
the Scythinians four stages, twenty parasangs, over a level plain, and they arrived
at some villages, and there remained for three days and collected provisions.

[19] From there they journeyed four stages, twenty parasangs, to a large and prosperous
inhabited city which was called Gymnias. From this city the ruler of the land sent
the Greeks a guide, in order to lead them through territory that was hostile to his
own.

[20] When the guide came, he said that he would lead them within five days to a place
from which they could see the sea; if he failed to do so, he was ready to accept death.
Thus taking the lead, as soon as he had brought them into the hostile territory, he
kept urging them to spread abroad fire and ruin, thereby making it clear that it {331}
was with this end in view that he had come, and not out of good-will toward the Greeks.

[21] On the fifth day they did in fact reach the mountain; its name was Theches. Now
as soon as the vanguard got to the top of the mountain, a great shout went up.

[22] And when Xenophon and the rearguard heard it, they imagined that other enemies
were attacking in front; for enemies were following behind them from the district
that was in flames, and the rearguard had killed some of them and captured others
by setting an ambush, and had also taken about twenty wicker shields covered with
raw, shaggy ox-hides.

[23] But as the shout kept getting louder and nearer, as the successive ranks that
came up all began to run at full speed toward the ranks ahead that were one after
another joining in the shout, and as the shout kept growing far louder as the number
of men grew steadily greater, it became quite clear to Xenophon that here was something
of unusual importance;

[24] so he mounted a horse, took with him Lycius and the cavalry, and pushed ahead
to lend aid; and in a moment they heard the soldiers shouting, "The Sea! The Sea!"
and passing the word along. Then all the troops of the rearguard likewise broke into
a run, and the pack animals began racing ahead and the horses.

[25] And when all had reached the summit, then indeed they fell to embracing one another,
and generals and captains as well, with tears in their eyes. And on a sudden, at the
bidding of some one or other, the soldiers began to bring stones and to build a great
cairn.

[26] Thereon they placed as offerings a quantity of raw ox-hides and walking-sticks
and the captured wicker shields; and the guide not only cut these {333} shields to
pieces himself, but urged the others to do so.

[27] After this the Greeks dismissed the guide with gifts from the common stock --
a horse, a silver cup, a Persian dress, and ten darics; but what he particularly asked
the men for was their rings, and he got a considerable number of them. Then he showed
them a village to encamp in and the road they were to follow to the country of the
Macronians, and, as soon as evening came, took his departure.

(text from the Perseus Project, Tufts University)