S. Baring-Gould, who visited Iceland in 1863, observed that if a tube be bent into two arms of unequal length, the shorter of which is closed, and if the tube be filled with water and the shorter arm then heated, all the characteristic phenomena of geyser action result, the water being finally ejected, with explosive violence from the longer tube. Now, it is probable that in nature each of these theories may find illustration, but it must still be acknowledged that in all cases Bunsen's theory is the partial explanation, and in many the only adequate one. The most superficial examination of the geysers in the Park will disclose two widely different characteristics as regards their external appearance and mode of eruption. On this basis they may be divided into two classes--the fountain geysers and the cone geysers. In the fountain geyser there is no cone or mound, but in its place a considerable pool which in intervals of rest bears perfect resemblance to the larger quiescent springs. The eruption generally consists of a succession of prodigious impulses by which vast quantities of water are thrown up one after another. There is ordinarily no continuous jet. To geysers of this class, Mackenzie's and Comstock's theories would seem to find closer application than to any others. Noted examples are the Fountain, the Great Fountain, the Grand and the Giantess Geysers. The cone geysers, on the other hand, have no pool about the crater, and water is not generally visible in the tube. There is always a self-built cone or mound of greater or less prominence, ranging from a broad gently-sloping mound, like that of Old Faithful, to a huge cone like that of the Castle. The eruptions from these geysers usually take the form of a continuous jet, and are more in accordance with the theory of Bunsen. Prominent examples are the Giant, the Castle, Old Faithful, the Lone Star, and the Union. [Illustration: _Terry Engr. Co._ _U. S. Geological Survey of the Territories._ Cone of the Giant Geyser.] [Illustration: _Terry Engr. Co._ _First sketch ever made._[AU]--_Trumbull._ Cone of Giant Geyser.] [AU] This sketch and a similar one of the Castle Geyser cone and two of the Yellowstone Falls are the _very first_ ever made of these objects. They were made in 1870 by Walter Trumbull, a member of the Washburn Party, and by Private Charles Moore, one of the escort under Lieutenant Doane. Moore was a man of excellent education and considerable culture, and it was a matter of comment among the members of the Expedition that he should be content with the condition of a private soldier. His quaint sketches of the Falls forcibly remind one of the original picture of Niagara made by Father Hennepin in 1697.