A few of the curved canals are also double.
There is one drawing in Mr.Lowell's book (p.

219) of the mouths, or starting points, of the Euphrates and Phison, two widely separated double canals diverging at an angle of about 40 deg.

from the same two oases, so that the two inner canals cross each other.

Now this suggests two wide bands of weakness in the planet's crust radiating probably from within the dark tract called the 'Mare Icarium,' and that some widespread volcanic outburst initiated diverging cracks on either side of these bands.

Something of this kind may have been the cause of most of the double canals, or they may have been started from two or more craterlets not far apart, the direction being at first decided by some local peculiarity of structure; and where begun continuing in straight lines owing to homogeneity or uniform density of material.