The differences in the shape and general arrangement of the cavities and tubes of the two caves, indicate that their action as geysers was very unlike. Wind Cave evidently sent a rather slender column to a great height, nearly perpendicular, and the water eruption was followed by a long steam period. Crystal Cave ejected a much larger jet more frequently, at a low angle of inclination, the eruption was sooner over, and was not followed by a steam period of any consequence. Thus it can be seen that the caves of the Black Hills prove the theories in regard to geyser action in Yellowstone Park, and those theories, in turn, prove the past history of the caves. The study of geyser action also shows that the conical or dome shape of some of the cave chambers is not due to the whirl of incoming floods, as in other regions, but to jets of water forced up from lower levels. Perhaps the finest geyser basin, and possible cave, ever in existence was destroyed when the Grand Canon of the Yellowstone became a canon. Evidences of the former conditions in control of this gorgeously brilliant scene are neither wanting nor doubtful. Steam constantly issues from numerous small vents in the canon walls, and a field glass reveals miniature geysers in action down in the depth of the canon, nearly half a mile below the top of the wall; while the entire canon shows, in both the color and character of its rocks, that chemical agencies have wrought changes here that have not been effected in other exposures of similar nature. It seems not improbable that the relation of Yellowstone River to the Grand Canon was the same as, at the present time, is that of the Firehole to the Upper, Middle, and Lower Geyser Basins: and that an explosion of great force was followed by a general collapse instead of the usual eruption of one of the grandest geysers; one result being the sudden precipitation of the river into a new, beautiful, and totally unexpected channel. After its great leap of two hundred and ninety-seven[6] feet at the Lower Fall, the river flows in a brilliant, narrow line of emerald green, broken by the white foam of frequent cascades, between magnificent walls of yellow, white, pink, and red of most vivid hues. FOOTNOTES: [6] Measurement by the Hayden Party.