72 GEOLOGICAL SUKVEY OF THE TEEEITOEIES. 



usually with the weclge-shai^ed end projecting upward, as if the mass 

 had slowly fallen down as the underlying rocks were worn away by 

 erosion. So thickly is this belt covered with these huge masses that it 

 is with the greatest difficulty one can walk across it. It would seem 

 that this bed must at one time have extended over, a portion or all 

 of 'the valley of Gardiner's Eiver. Much of the rock is very compact, 

 and would make beautiful building-stone, on account of its close texture 

 and color, and it could be converted into the whitest of lime. If the 

 rocks are examined, however, over a considerable area, they will be 

 found to possess all the varieties of structure of a hot-spring deposit. 

 Some portions are quite spongy, and decompose readily ; others are made 

 up of very thin laminse, regular or wavy ; enough to show the origin of 

 the deposit without a doubt. But in w^at manner w^as it formed? 1 

 believe that the limestone was precipitated in the bottom of a lake, 

 which was filled with hot springs, much as the calcareous matter is laid 

 down in the bottom of the ocean at the present time. Indeed, portions 

 of the rock do not differ materially from the recent limestones now form- 

 ing in the vicinity of the "West India Islands. The deposit was evi- 

 dently laid down on a nearly level surface, with a moderately uniform 

 thickness, and the strata are horizontal. Since this group of strata 

 was formed, the country has been elevated to some extent at least, and 

 the valley of Gardiner's Eiver has been carved out, so that the com- 

 mencement of the period of activity of these springs must date back to 

 a period merging on to, but just prior to, the present, probably at the 

 time of the greatest action of the volcanic forces. 



We may now ask why these deposits are mainly calcareous, and what 

 is the source of the lime. 



I have already given a brief account of the geological formations in 

 the immediate vicinity. On the side of Gardiner's Eiver, opposite the 

 hot springs, there is a bluff wall extending about six miles, composed 

 of 150 feet in the aggregate of Upper Cretaceous and Lower Tertiary 

 strata, with some irregular intercalated beds of basalt. The river itself 

 flows through a sort of monoclinal interval ; that is, the bluff" wall just 

 alluded to is formed of the outcropping edges of the strata, while on 

 the op})Osite side or slope the lower beds incline in the same direction. 

 Near the river some of the lower beds are Cretaceous, but they soon pass 

 to the Jurassic and Carboniferous ; on the east side of the springs are beds 

 of arenaceous limestone full of Jurassic fossils. We can then see that the 

 vast thickness of Tertiary and Cretaceous strata once extended across 

 Warm Spring Creek, over the slope of the mountain occupied by the 

 hot-spring deposit, and, probably, westward across the vast divide into 

 the Missouri Valley. We have,'also, clear proof that, underneath this 

 calcareous deposit, there is at least a thickness of 1,500 feet of Carbon- 

 iferous limestone. 



If the origin of the heat which so elevates the temperature of the 

 waters of these springs is as deep-seated as is generally supposed, then 

 the heated waters have ample play for their jiower in dissolving the 

 calcareous rocks beneath. There are several localities in the valley of 

 the Yellowstone where the deposits are calcareous, but most of. them 

 are unimportant, and the springs themselves have entirely disappeared. 

 If we divide the springs according to the character of their deposits, 

 we shall find that there are two principal classes — those in which lime 

 predominates, and those which have an excess of •silica; or calcareous 

 and siliceous springs. We shall present this subject more fully in a 

 subsequent portion of this report. 



In figure 21 1 have attempted to present an ideal section of the strata 



