"The difference between geysers and ordinary hot springs is not readily explained, nor even always recognized. The difference between a quiet thermal spring and a geyser in active eruption is very marked, but between the two there is every grade of action. Some geysers appear as quiet springs, as for instance the Grand Geyser during its period of quiescence. Others might easily be mistaken for constantly boiling springs, as in the case of the Giant Geyser, in which the water is constantly in active ebullition. This is true also of the Strockr of Iceland. Many of the springs, therefore, that in the Yellowstone Park have been classed as constantly boiling springs may be unsuspected geysers. The Excelsior Geyser was not discovered to be a geyser until eight years after the setting aside of the park. Almost all constantly boiling springs have periods of increased activity, and those which spurt a few feet into the air have been classed as pseudo-geysers. "It has been noticed that geysers occur where the intensity of volcanic action is decreasing. In the neighborhood of active volcanoes, such as Vesuvius, the temperature appears to be too high, and the vapor escapes as steam from what are called stufas. When the rocks at the surface are more cooled the water comes forth in liquid form. "We will now pass to the various geyser theories that have been proposed by different writers." Dr. Peal then proceeds to give the theories of Sir J. Herschell and Sir George McKenzie, but as they are accepted and extended by others, we may pass on to Bischof's, of which Dr. Peal says: "Very similar to McKenzie's theory is the one adopted by Bischof in his Researches on the Internal Heat of the Globe (pages 227, 228). It is really the theory of Krug Von Nidda, who examined the geyser in 1833. Bischof says: