will have to be constructed, and various modifications made in the tubes until results are reached analogous to those seen in natural geysers.  If water in a glass tube be heated with rapidity from the bottom, it will be expelled from the tube violently, and if boiled in a kettle which has a lid and a spout, either the lid will be blown off or the water will be forced out through the spout.  The first case is an illustration, in part at least, of Bunsen’s theory, and the second exemplifies the theories which presuppose the existence of subterranean cavities with tubes at or near the surface.  According to the former we must suppose that the layer of rock, extending seventy-five to seventy-seven feet below the surface, contains sufficient heat to account for geyseric phenomena; or else that the geyser tube has some opening, either at the bottom or on the sides, by which steam and superheated water have access to it from a considerably greater depth where the temperature is very high.  At these depths caverns probably exist.” * * * *

“That such cavities exist is more than probable.  On page 405 I have indicated my belief, that all geysers are originally due to a violent outburst of steam and water, and under such conditions, irregular cavities and passages are more likely to be formed than regular tubes.” * *

“In view of what we have just written, Bunsen’s conclusion (No. 2) would have to be modified somewhat.  His conclusion was that at no point in the tube did the water attain the temperature of ebullition which it should have under the pressure to which it is subjected.  As far as this relates to the straight tube in which his temperatures were taken, it may be so; but if he could have taken temperatures in the side conduit, I have little doubt he would soon have reached a point where the temperature would not only be at the boiling point for that depth but even exceed it.  In the Yellowstone Park we obtained a number of surface temperatures which were above the boiling point.  In the Great Geyser of Iceland, the mass of water in the tube prevents this condition at the surface, and when it takes place opposite the aperture an eruption is caused.  In the main, however, I am inclined to accept Bunsen’s theory, especially as it seems to me to require subterranean cavities in which the water must be heated.  Whether these are caverns, enlargements of tubes, or sinus channels, appears to me to be of no consequence, except as the interval or period of the geyser might be affected by the form of the reservoir holding the water.”

Dr. Peal has reached conclusions which present an imaginary picture of the interior structure of the great geysers of the Park, that bears a striking resemblance to what the two caves of the Black Hills prove to be the true conditions; although it is evident he had in mind caverns of no such vast extent, nor of so complicated a system of cavities and tubes.  He overlooked an important feature, however, in not accepting