72 REPORT UNITED STATES GEOLOGICAL SURVEY. 



deposits, on wliicli the springs are at present found, occupy about 170 

 acres. They extend over two miles back from the river, and the upper 

 springs are 1,000 feet above its level. On the summit of a long, ridge- 

 like mountain which extends south from the head of the gulch in which 

 the springs are situated, is found also a thick deposit of old spring ma- 

 terial, covering an area of about three-quarters of a square mile. An- 

 other area, including about 90 acres of old deposits, is found between 

 the forks of Gardiner's Eiver, southeast of the main springs. These 

 remnants prove that formerly the springs at this locality must have 

 extended over a much larger area than they do at present. These old 

 deposits will be referred to again. 



History. — These springs were not visited by the expeditions of 1869 

 and 1870, to the members of which they were unknown, because they 

 followed the Yellowstone Eiver, and, therefore, passed the mouth of 

 Gardiner's Eiver without seeing them. In 1871, when we first visited 

 them, we found that several cabins had been erected, and that two men 

 from Bozeman, Mont., had entered a claim to the springs, while a 

 number of invalids were using the waters with benefit. They called the 

 springs Mammoth Hot Springs. The first description ever published 

 was that in the report of the survey for 1871. 



Geology.— The geology of this region will be fully given in Mr. 

 Holmes's report, and I will therefore only refer to it briefly. The rocks 

 upon which the deposits are found are referable to the Middle and 

 Lower Cretaceous and Jurassic formations. Ehyolite is found east of 

 the springs, on the north side of the gulch, where it is in connection 

 with the Jurassic. It is also found on the summit of the long, ridge-like 

 hill, where it is capped with old hot spring deposits. Beneath the 

 Jurassic rocks there are probably Carboniferous limestones, in which 

 we have the source of the carbonic acid and carbonates that are found 

 in the water of the springs. The comparatively low temperatures 

 found here are also explained by the presence of sedimentary rocks, the 

 water in passing through them losing a great portion of its original 

 heat. South of the springs the local drift obscures the underlying rocks, 

 but it is probable that they are mainly Jurassic and Cretaceous here as 

 on the north side. 



The springs at present are found mainly on two masses of deposit,- 

 and these are arranged in terraces. Of these there are four principal 

 ones, which have minor subdivisions. Tlie map shows the main divisions, 

 which have a remarkable resemblance to each other. The lower mass, 

 on which the most active springs are now situated, has at its foot two 

 cones or isolated masses of deposit, and is fringed along its front with 

 basins. The upper mass has a similar front, but its springs now are 

 few and feeble in their action. Back of this upper mass is an older one 

 still, at the foot of which are several old cones, which are shown in the 

 left-hand lower corner of the map. This old upper mass is overgrown 

 with trees, but the ruins of old basins can still be detected, almost 

 covered with soil, and much broken down. l!^ortheast of Lookout Hill 

 another old terrace is found, with remnants of basins on its front. It is 

 probable that at some time in the past springs were active on all the 

 terraces, not perhaps simultaneously, but successively. 



In the report for 1872 I divided the terraces into fourteen for the sake 

 of comparison. As a map accompanies the present report, this will be 

 unnecessary, and I will designate the springs by number, and in the de- 

 scription give their identification with the numbers and terraces of 1872, 

 as far as can be done. The changes that have occurred since that time 

 will be noted after I have given the descriptions of the different springs 

 that are included in the following table: 



