in Thoreau’s Journal:

From the hill, I look westward over the landscape. The deciduous woods are in their hoary youth, every expanding bud swaddled with downy webs. From this more eastern hill, with the whole breadth of the river valley on the west, the mountains appear higher still, the width of the blue border is greater, —not mere peaks, or a short and shallow sierra, but a high blue table-land with broad foundations, a deep and solid base or tablet, in proportion to the peaks that rest on it. As you ascend, the near and low hills sink and flatten into the earth; no sky is seen behind them; the distant mountains rise. The truly great are distinguished. Vergers, crests of the waves of earth, which in the highest break at the summit into granitic rocks over which the air beats. A part of their hitherto concealed base is seen blue. You see, not the domes only, but the body, the facade, of these terrene temples. You see that the foundation answers to the superstructure. Moral structures. (The sweet-fern leaves among odors now.) The successive lines of haze which divide the western landscape, deeper and more misty over each intervening valley, are not yet very dense; yet there is a light atmospheric line along the base of the mountains for their whole length, formed by this denser and grosser atmosphere through which we look next the earth, which almost melts them into the atmosphere, like the contact of molten metal with that which is unfused; but their pure, sublimed tops and main body rise, palpable skyland above it, like the waving signal of the departing who have already left these shores. It will be worth the while to observe carefully the direction and altitude of the mountains from the Cliffs. The value of the mountains in the horizon,  —would not that be a good theme for a lecture? The text for a discourse on real values, and permanent; a sermon on the mount. They are stepping-stones to heaven, —as the rider has a horse-block at his gate, by which to mount when we would commence our pilgrimage to heaven; by which we gradually take our departure from earth, from the time when our youthful eyes first rested on them, —from this bare actual earth, which has so little of the hue of heaven. They make it easier to die and easier to live. They let us off.