Occasionally, says Lieutenant Doane, this anomaly is seen, namely: "two springs, at different levels, both boiling violently; one pours a large and constant stream into the other, yet the former does not diminish, nor does the latter fill up and overflow." Most of the springs, however, seem to be independent of each other, since they have different levels at the surface, different temperatures and pulsations, and rarely are the waters and deposits of any two exactly alike. Passing northward through dense woods and almost impenetrable fire-slashes, the next noteworthy region arrived at is the valley of Bridge Creek, the creek receiving its name from a natural bridge of trachyte thrown across the stream. The bridge is narrow, affording scanty room for the well-worn elk-trail two feet wide, while the descent on either hand is so great that a fall from the bridge would be fatal to man or beast. Numerous herds of elk make daily use of this convenient passway. Dead and dying springs are abundant all along the valley of this creek, the most of them being reduced to mere steam-vents. In one place the spring deposits cover several acres and present a most attractive picture. The ground is thickly covered with conical mounds, from a few inches in diameter to a hundred feet, full of steaming orifices lined with brillant sulphur-crystals. The under side of the heated crust is everywhere adorned in the same manner. The basis of the deposit is snow-white silica, but it is variegated with every shade of yellow from sulphur, and with scarlet from oxide of iron. From a distance the whole region has the appearance of a vast lime-kiln in full operation. Most of the country has been eroded into rounded hills from fifty to two hundred feet high, composed of the whitish-yellow and pinkish clays and sands of the modern lake deposit, which seems to prevail more or less all round the rim of the basin, reaching several hundred feet above the present level of the lake. Between Bridge Creek and the outlet of the lake, completing the circuit of the basin, is the Elephant's Back, a long, low mountain, noticeable only for its rounded summit and precipitous sides. CHAPTER XII. UPPER GEYSER BASIN OF FIREHOLE RIVER. Just over the western margin of the Yellowstone Basin, yet within the limits of our great National Park, is the grand geyser region of Firehole River. Here, in a valley a dozen miles long and two or three wide, is an exhibition of boiling and spouting springs on a scale so stupendous that if all the corresponding phenomena of all the rest of the world could be brought into an equal area the display would seem as nothing in comparison.