The boiling spring is intermediate between the quiescent spring and the geyser. The circulation is sufficiently free to prevent a great rise of temperature in the lower depths of the tube, and nothing more than a surface ebullition, often extremely violent, results. These springs are generally objects of secondary interest. They are simply enormous caldrons; any kettle placed over a brisk fire simulates their action on a small scale. The mud springs, or Paint Pots, as they are now always called, are extremely curious phenomena. They are caused by the rising of steam through considerable depths of earthy material. The water is just sufficient in quantity to keep the material in a plastic condition, and the steam operates upon it precisely as it does upon a kettle of thick mush. Generally there are various mineral ingredients, mostly oxides of iron, which impart different colors to different parts of the group. As the steam puffs up here and there from the thick mass, it forms the mud into a variety of imitative figures, prominent among which is that of the lily. These figures immediately sink back into the general mass, only to be formed anew by other puffs of steam. The material is so fine as to be almost impalpable between the fingers. Lieutenant Doane, however, justly observes that "mortar might well be good after being constantly worked for perhaps ten thousand years." Other phenomena very common throughout the Park are steam vents or fumaroles in which there is no water or only a very small quantity. They are not ordinarily of much popular interest, although there are a few remarkable examples. Among these may be mentioned the Black Growler in the Norris Geyser Basin, and Steamboat Spring on the east shore of the Yellowstone Lake. The hot spring areas of the Park are both numerous and extensive. They abound throughout the valleys of the Yellowstone, the Madison, and the Snake Rivers, and the number of individual springs is several thousand. CHAPTER V. FOSSIL FORESTS OF THE YELLOWSTONE. A region of great popular and scientific interest in the Yellowstone Park, although as yet hardly known to the tourist, owing to the incomplete condition of the road system, is that of the Fossil Forests in the north-east corner of the Park. The facts which have been brought to light concerning the origin of these forests are worthy of particular consideration.