OUR NATIONAL PARKS. 15 



now regarded with wonder by so many, is simple. Water from the 

 surface trickling through cracks in the rocks, or water from subter- 

 ranean springs collecting in the bottom of the geyser's crater, down 

 among; the strata of intense heat, becomes itself intensely heated and 

 gives off steam, which expands and forces upward the cooler water 

 that lies above it. 



It is then that the water at the surface of the geyser begins to bub- 

 ble and give off clouds of steam, the sign to the watchers above that 

 the geyser is about to play. 



At last the water in the bottom reaches so great an expansion 

 under continued heat that the less heated water above can no longer 

 weigh it down, so it bursts upward with great violence, rising many 

 feet in the air and continuing to play until practically all the water 

 in the crater has been expelled. The water, cooled and falling back 

 to the ground, again seeps through the surface to gather as before in 

 the crater's depth, and in a greater or less time, according to diffi- 

 culties in the way of its return, becomes reheated to the bursting 

 point, when the geyser spouts again. 



One may make a geyser with a test tube and a Bunsen burner. 



THE HOT WATER PHENOMENA 



Nearly the entire Yellowstone region, covering an area of about 

 3,300 square miles, is remarkable for its hot-water phenomena. The 

 geysers are confined to three basins lying near each other in the 

 middle west side of the park, but other hot water manifestations 

 occur at more widely separated points. Marvelously colored hot 

 springs, mud volcanoes, and other strange phenomena are frequent. 

 At Mammoth, at Norris, and at Thumb the hot water has brought 

 to the surface quantities of white mineral deposits which build 

 terraces of beautifully incrusted basins high up into the air, often 

 engulfing trees of considerable size. Over the edges of these carved 

 basins pours the hot water. Microscopic plants called algae grow on 

 the edges and sides of these basins, assisting the deposition of the 

 mineral matter and painting them hues of red and pink and bluish 

 gray, which in warm weather glow brilliantly, but in cold weather 

 almost disappear. At many other points lesser hot springs occur, 

 introducing strange, almost uncanny, elements into wooded and 

 otherwise quite normal landscapes. 



A tour of these hot-water formations and spouting geysers is an 

 experience never to be forgotten. Some of the geysers play at quite 

 regular intervals. For many years the celebrated Old Faithful played 

 with great regularity every seventy minutes, but during the summer 

 of 1915 the interval lengthened to about eighty-five minutes, due, it 

 is supposed, to the smaller snowfall and consequent lessened water 

 supply of the preceding winter. Some of the largest geysers play at 



