Suppose a little pool with perpendicular sides, say twenty feet across. It leaps and boils two feet high. It deposits nothing till the water comes to the cooling edge. Then it builds up a wall where it overflows, and wherever it flows it builds. The result is that you walk up the gentle slopes of a broad flat cone, and find the little lakelet in a gorgeous setting, perfectly full at every point of the circumference. If there is but little overflow, the result may be to deposit all the matter where it first cools, and make a perpendicular wall around the cup two or ten feet high. If the overflow is too much to be cooled at once, the deposit may still be made fifty or one hundred feet from the point of issue. If the overflow is sufficient, it may be building up every inch of a vast cone at once, every foot being wet. [Illustration: The Punch Bowl, Yellowstone Geysers.] Many minerals are held in solution and are deposited at various stages of evaporation. Let us suppose the lake to have the bottom sloping toward the abysmal center; the different minerals will be assorted as if with a sieve. At the Sunlight Basin the edge is as flaming red as one ever sees in the sunlit sky. And every color ever seen in a sunset flames almost as brilliantly in the varying depths. Suppose a low cone to be flooded only occasionally, as in the case of the Old Faithful geyser. The cooled water falling from the upper air builds up, under the terrible drench of the cataract, walls three or four inches high, making pools of every conceivable shape, a few inches deep, in which are the most exquisite and varied colors ever seen by mortal eye. You walk about on these dividing walls and gaze into the beaded and impearled pools of a hundred shades of different colors, never equaled except by that perpetual glory of the sunset. Consider the case of a pool that does not overflow. Just as lakes that have no outlet must grow more and more salt till some have become solid salt beds, so must this pool, tossing its hot waves two or three feet high, evaporate its water and deposit its solids. Where? First, against the cooler sides of the rock under the water, tending to reduce the opening to a mere throat. Second, each wavelet tossed in air is cooled, and deposits on the edge, solid as quartz, a crust that overhangs the pool and tends to close it over as with hot ice. It may build thus a mound fifteen feet high with an open throat in the middle. Thus the pool has constructed an intermittent geyser. If the water supply continues, it also destroys itself. The throat closes up by its own deposits. It is a case of geyseral membranous croup.