YELLOWSTONE Geological History of the Yellowstone National Park

The evidence of the antiquity of the hot spring deposits is, perhaps, shown in an equally striking manner and by a wholly different process of geological reasoning. Terrace Mountain is an outlying ridge of the rhyolite plateau just west of the Mammoth Hot Springs. It is covered on the summit with thick beds of travertine, among the oldest portions of the Mammoth Hot Springs deposits. It is the mode of occurrence of these calcareous deposits from the hot waters which has given the name to the mountain. Lying upon the surface of this travertine on the top of the mountain are found glacial bowlders brought from the summit of the Gallatin Range, 15 miles away, and transported on the ice sheet across Swan Valley and deposited on the top of the mountain, 700 feet above the intervening valley. It offers the strongest possible evidence that the travertine is older than the glacier which has strewn the country with transported material. How much travertine was eroded by the ice is, of course, impossible to say, but so friable a material would yield readily to glacial movement.

Still another method of arriving at the great antiquity of the thermal energy and the age of the hot spring formation is by determining the rate of deposition and measuring the thickness of the accumulated sinter. This method, although the one which would perhaps first suggest itself, is, in my opinion, by no means as satisfactory as the geological reasoning already given. It is unsatisfactory because no uniform rate of deposition can be ascertained for even a single area, like the Upper Geyser Basin, and it is still more difficult to arrive at any conclusion as to the growth of the sinter in the past. Moreover, it is quite possible that heavy deposits may have suffered erosion before the present sinter was laid down. It however corroborates other methods and possesses the advantage of being a direct way.

It may be well to add that there exists the greatest contrast between the deposits of the Mammoth Hot Springs and those found upon the plateau. At the Mammoth Springs they are nearly pure travertine, with only a trace of silica, analyses showing from 95 to 99 per cent of calcium carbonate. On the plateau, the deposits consist for the most part of siliceous sinter, locally termed "geyserite." The reason for the difference is this: At the Mammoth Hot Springs the steam, although ascending from fissures in the igneous rock, comes in contact with the waters found in the Mesozoic strata, which here form the surface rocks. The Jura or Cretaceous limestones have furnished the lime held in solution and precipitated on the surface as travertine. On the other hand, the mineral constituents of the plateau waters are derived almost exclusively from the highly acidic lavas, which carry but a small amount of lime.

TERRACES AT MAMMOTH HOT SPRINGS.

Deposition of sinter from the hot waters of the geyser basins depends in a great measure on the amount of silica held in solution, which varies considerably at the different localities and may have varied still more in past time. The silica, as determined by analyses, ranges from 0.22 to 0.60 grammes per kilogram of water, the former being the amount found in the water of the caldron of the Excelsior Geyser and the latter at the Coral Spring in the Norris Basin. Analysis shows that from one-fifth to one-third of the mineral matter held in solution consists of silica, the remaining constituents being readily soluble salts carried off by surface drainage. A few springs highly charged with silica, like the Coral, deposit it on the cooling of the waters; but such springs are exceptional. At most springs and geysers it results after evaporation, and not from mere cooling of the water. It seems probable that the nature and amount of alkaline chlorides and carbonates present influence the separation of silica. Temperature also may in some degree influence the deposition. My friend, Mr. Elwood Hofer, has called my attention to an observation of his made in midwinter, while on one of his snowshoe trips through the park. He noticed that certain overflow pools of spring water, upon being frozen, deposited a considerable amount of mineral matter. He has sent me specimens of this material, which, upon examination, proved to be identical with the silica deposited from the Coral Spring upon the cooling of the water. Demijohns of geyser water which have been standing for one or two years have failed to precipitate any silica. Quite recently, in experimenting upon these waters in the laboratory, it was noticed that on reducing them nearly to the freezing point no change took place, but upon freezing the waters there was an abundant separation of free silica. The waters frozen in this way were collected from the Coral Spring, Norris Basin, and the Taurus Geyser, Shoshone Basin.

ALGAE BASINS.

Again, there is no doubt that the algous growths flourishing in the hot waters of the park favor the secretion of silica and calcic carbonate and exert a potent influence in building up both the sinter and travertine deposits far greater than one might at first be led to suppose. These processes of assimilation are steadily taking place without interruption, as all algae act as geological agents. The silica and lime brought to the surface by hot springs is, upon the death of the algae, transformed into sinter and travertine, becoming rock masses, which later show scarcely any sign of their origin from plant life. Tourists are seldom aware that the harmonious and brilliant tints are due to vegetable growths. Algae develop equally well in the waters of all geyser basins and upon the terraces of Mammoth Hot Springs. Water boils in the Upper Geyser Basin at 198° F., and rudimentary organisms appear at about 185° F., although no definite line can be drawn beyond which all life ceases. These low vegetable organisms occur in nearly all pools, springs, and running water upon the plateau. Wherever boiling waters cool to the latter temperature algae make their appearance, and with the lowering of temperature on exposure to air still more highly organized forms gradually come in. It is believed that at about 140° F. conditions are favorable for a rapid development of numerous species. Many forms of algae flourish within restricted ranges of temperature, and the different species possess characteristic colors and habits of growth dependent upon such changes of temperature. After a little experience it is quite possible, upon noting the nature of the plant life, to make a sure guess as to the temperature of the water in which the species grow. As water in the geyser pools and caldrons frequently stands at or near the boiling point, no life exists at the centers of discharge, but with a rapid lowering of temperature algae appear, with corresponding changes of color, in the shallow pools and overflow channels. In the geyser basins the first evidence of vegetation in an overflow stream consists of creamy white filamentary threads, passing into light flesh tints, then to deep salmon. With distance from the source of heat the prevailing colors pass from bright orange to yellow, yellowish green, and emerald, and in the still cooler waters various shades of brown. This, of course, is a simple statement of phenomena which really display highly complex conditions. No two pools or overflow channels are quite alike in their occurrence either as regards flow of water or development of algae.

Several methods have been devised for ascertaining the growth of deposition of the geyserite. One way is by allowing the water to trickle over twigs, dried grasses, or almost anything exposing considerable surface, and noting the amount of incrustation. This way gives the most rapid results, but is far from satisfactory and by no means reproduces the conditions existing in nature. Other methods employed are placing objects on the surface of the water or, still better, partially submerging them in the hot pools, or again by allowing the water to run down an inclined plane with frequent intervals for evaporation and concentration.

BOWL OF GREAT FOUNTAIN GEYSER, LOWER GEYSER BASIN.

The vandals who delight to inscribe their names in public places have invaded the geyser basins in large numbers and left their addresses upon the geyserite in various places. It is interesting to note how quickly these inscriptions become indelible by the deposition of the merest film of silica upon the lead-pencil marks, and, at the same time, how slowly they build up. Names and dates known to be 6 and 8 years old remain perfectly legible, and still retain the color and luster of the graphite. That there is some increase in the thickness of the incrustation is evident, although it grows with incredible slowness. Mr. Weed tells me that he has been able, in at least one instance, to chip off this siliceous film and reproduce the writing with all its original distinctness, showing conclusively that a slow deposition has taken place. Pencil inscriptions upon the siliceous sinter at Rotomahana Lake, in New Zealand, are said to be legible after the lapse of 20 or 30 years. It is easy to see that various ingenious devices might be planned to estimate the rate of deposition, but in my opinion none of them equal a close study of the conditions found in nature, especially where investigations of this kind can be watched from year to year. All observations show an exceedingly slow building up of the geyserite formation. This is well seen in the repair going on where the rims surrounding the hot pools have been broken down, and where it might be supposed that the building-up process was under the most favorable conditions; yet, in a number of instances, I can see no appreciable change in three or four years. Revisiting hot springs in out-of-the-way places after several years' absence, I am surprised to see that objects that I had noted carefully at the time remain unchanged. Taking the entire area of the Upper Geyser Basin covered by sinter, I believe that the development of the deposit does not exceed one-thirtieth of an inch a year, and this estimate I believe to be much nearer the maximum than the minimum rate of growth. Supposing the deposit around Castle Geyser to have been built up with the same slowness as observed to-day, and assuming it to grow at the rate given—one-thirtieth of an inch a year—it would require over 25,000 years to reach its present development. This gives us a great antiquity for the geyserite, but I believe that the deposition of the siliceous sinter in the park has been going on for a still longer period of time. It is certain that the decomposition of the rhyolite of the plateau dates still further back.

From a geological point of view, there is abundant evidence that thermal energy is gradually becoming extinct. Tourists revisiting the park after an absence of two or three years occasionally allude to the springs and geysers as being less active than formerly and as showing indications of rapidly dying out. It is true that slight changes are constantly taking place, that certain springs become extinct or discharge less water, but this action is fully counterbalanced by increased activity in other localities. Close examination of the source of the thermal waters fails to detect any diminution in the supply. Moreover, it stands to reason that if the flow of these waters dated—geologically speaking—far back into the past, the few years embraced within the historical records of the park would be unable to indicate any perceptible change based upon a gradual diminution of the heat.

CONE OF CASTLE GEYSER, UPPER GEYSER BASIN.

The number of geysers, hot springs, mudpots, and paintpots scattered over the park exceeds 3,000, and if to these be added the fumaroles and solfataras, from which issue in the aggregate enormous volumes of steam and acid and sulphur vapors, the number of active vents would in all probability be doubled. Each one of these vents is a center of decomposition of the acid lavas. The following list comprises the principal geysers known in the Norris, Lower, and Upper Geyser Basins.

A comparative study of the analyses of the fresh rhyolite, the various transition products, and the thermal waters points clearly to the fact that the solid contents of these waters are derived for the most part from the volcanic rocks of the plateau. During the progress of the work of the Geological Survey in the Yellowstone Park there have been collected from many of the more important localities samples of the waters, which have been subjected to searching chemical analyses in the laboratory of the survey, by Messrs. F. A. Gooch and J. E. Whitfield.

They are all siliceous alkaline waters holding the same mineral constituents, but in varying qualities. Silica forms the principal deposit, not only immediately around the springs but, over the entire floor of the basins. The carbonates, sulphates, chlorides, and traces of other easily soluble salts are carried off in the waters. Oxides of iron and manganese and occasionally some calcite occur under certain conditions in the caldrons of the hot springs or immediately around their vents.

Concentrations from large quantities of these waters fail to show the presence of even a trace of copper, silver, tin, or other metal. Nearly all the waters carry arsenic, the amount present, according to Messrs. Gooch and Whitfield, varying from 0.02 to 0.25 per cent of the mineral matter in solution.

Among the incrustations found at several of the hot springs and geysers is a leek-green amorphous mineral, which proves on investigation to be scorodite, a hydrous arseniate of iron. The best occurrence observed is at Josephs Coat Springs, on the east side of the Grand Canyon of the Yellowstone, where it occurs as a coating upon the siliceous sinter lining the caldron of a boiling spring. Analysis shows a nearly pure scorodite, agreeing closely with the theoretical composition:

Alteration of the scorodite into limonite takes place readily, which in turn undergoes disintegration by the wearing of the water, and is mechanically carried away. So far as I know, this is the only occurrence where scorodite has been recognized as deposited from the waters of thermal springs. Although pure scorodite is only sparingly preserved at a few localities in the Yellowstone Park, it is easily recognized by its characteristic green color, in strong contrast with the white geyserite and yellow and red oxides of iron. After a little practice the mineral green of scorodite is not easily mistaken for the vegetable green of the algeous growths. The latter is associated everywhere with the hot waters, while the former, a rare mineral, is obtained only in small quantities after diligent search. In America traces of arsenic have been reported from several springs in Virginia, and quite recently sodium arseniate has been detected in the hot springs of Ashe County, N. C. Arsenical waters of sufficient strength to be beneficial for remedial purposes and not otherwise deleterious are of rare occurrence. In France the curative properties of arsenical waters have been long recognized, and the famous sanitarium of La Bourboule, in the volcanic district of the Auvergne, has achieved a wide reputation for the efficacy of its waters in certain forms of nervous diseases. Hygeia Springs carries 0.3 of a grain of sodium arseniate to the gallon. The Yellowstone Park waters, while they carry somewhat less arsenic than those of La Bourboule, greatly exceed the latter in their enormous overflow. It is stated that the entire discharge from the springs of La Bourboule amounts to 1,500 gallons per minute. The amount of hot water brought to the surface by the hot springs throughout the park is by no means easily determined, although during the progress of investigations I hope to make an approximate estimate. According to the most accurate measurements which could be made, the discharge from the caldron of the Excelsior Geyser amounted to 4,400 gallons of boiling water per minute. The sample of the Excelsior Geyser water collected August 25, 1884, yielded 0.19 grain of sodium arsenate to the gallon. It is impossible to say as yet what curative properties these park waters may possess in alleviating the ills of mankind. Nothing but an extended experience under proper medical supervision can determine.

Changes modifying the surface features of the park in recent times are mainly those resulting from the filling up with detrital material of the valleys and depressions worn out by glacial ice, and those produced by the prevailing climatic conditions. Between the park country and what is known as the arid regions of the West there is the greatest possible contrast. Across the Central Plateau and the Absaroka Range the country presents a continuous mountain mass 75 miles in width, with an average elevation unsurpassed by any area of equal extent in the northern Rocky Mountains. It is exceptionally situated to collect the moisture-laden clouds, which coming from the southwest precipitate immense quantities of snow and rain upon the cooled tableland and neighboring mountains. The climate in many respects is quite unlike that found in the adjacent country, as is shown by the meteorological records, the amount of snow and rainfall being higher, and the mean annual temperature lower. Rainstorms occur frequently throughout the summer, while snow is quite likely to fall any time between September and May. Protected by the forests the deep snows of winter lie upon the plateau well into midsu mer, while at still higher altitudes, in sheltered places, it remains throughout the year. By its topographical structure the park is designed by nature as a reservoir for receiving, storing, and distributing an exceptional water supply, not exceeded by any area near the headwaters of the great continental rivers. The Continental Divide, separating the waters of the Atlantic from those of the Pacific, crosses the park plateau from southeast to northwest. On both sides of this divide lie several large bodies of water, which form so marked a feature in the scenery of the plateau that the region has been designated the lake country of the park. Yellowstone Lake, the largest lake in North America at this altitude (7,740 feet) and one of the largest in the world at so high an elevation above sea level, presents a superficial area of 139 square miles, and a shore line of nearly 100 miles. From measurements made near the outlet of the lake in September, 1886, the driest period of the year, the discharge was found to be 1,525 cubic feet per second, or about 34,000,000 imperial gallons per hour.