Though I indulged in a good deal of speculation as to the height of the dunes, I was unable to estimate their altitude with any degree of accuracy, for there was absolutely nothing with which to compare them. We saw a few quite imposing hills, which in certain localities would be called mountains, and which, so the guide told us, enjoyed names of their own; but, generally speaking, I imagine that the dunes are by no means as high as they appear in that wonderfully clear air—probably not much over four hundred or five hundred feet as a rule, though I have since been told that some of them are six hundred feet in height, which is considerably higher than the Washington Monument. Because, in our school geographies, the deserts were invariably tinted saffron, we have become accustomed to speak of them as “yellow.” But the Sahara, though frequently tawny in spots, runs through the whole gamut of colors. In the early morning it is a dirty bluish gray, of much the same tone as the refuse from a soda-ash manufactory; but as the sun rises it becomes a dazzling white, like drifted snow, so glaring that the eyes must be protected with tinted glasses. Under certain atmospheric conditions, however, I have seen the outcropping rocks of the _hammada_ become as red as the walls of the Grand Cañon. But the desert assumes its loveliest tints with the approach of nightfall, when it gradually changes from white to vivid orange, to blue, to amethyst, to deepest purple. Then, when the stars come out, it changes to gray again, an indescribably soft and misty gray, like smoky chiffon over silver tissue.