Why it should take this form is one of the most pregnant problems about the planet. For it is the most artificial-looking phenomenon of an artificial-looking disk.
III. SPOTS IN THE DARK REGIONS.
To return now from these outposts of investigation to our main subject-matter, and to another phenomenon of more recent discovery than the double canals, and yet more suggestive of interpretation. We have seen what shows at one end of the canals, their inner end; namely, the oasis. But it seems that there is also something exceptional at the other. At the mouth of each canal, at the edge of the so-called seas, appears a curious dark spot, of the form of a half-filled angle; the sort of a mark with which one checks items on a list. Its form is singularly appropriate, according to mundane ideas, for it appears before the canal itself is visible, as if to mark the spot where the canal will eventually be. It lies in the so-called seas, and looks to be of the same color as they, but deeper in tint.
All the canals that debouch into the dark regions are provided with these terminal triangles, except those that lead out of long estuaries, like the Nilosyrtis, the Hiddekel, the Gihon, and so forth. The double canals are provided with twin triangles. That the triangular patches are phenomena connected with