A glance at any accurate maps will show that water-courses in nearly all regions are so disposed .that they are somewhat regularly spaced, the intervals between their channels of like size being approximately the same even when the character of the rocks and the amount of the rainfall are somewhat varied. My attention was called to this fact some 20 years ago, when considering the relations of the rivers of Kentucky. While comparing the order of the under and over ground channels of that district I noted that while the cavern waters followed no distinct order in their placement, except such as was determined by the jointing of the limestone rocks in which they were excavated, the surface streams of the neighboring cavernless country were grouped with rather definite intervals which did not distinctly vary, whatever the character of the subjacent rock might be. . . .