384 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



It appears from this that even in volcanic rocks, where the tempera- 

 ture is greater than in other places, there are variations. 



From what has been written in the preceding pages the following 

 conclusions may be drawn as to the source of heat in thermal springs : 



1st. According to the ideas uniformly received at the present time 

 the heat of thermal springs is due to the internal heat of the earth, 

 which meteoric waters acquire by penetrating the rocks, after which 

 they come to the surface as thermal springs. 



2d. Ordiuarily this heat is simply due to the regular downward in- 

 crease of heat in the earth (about 1° to 50 or 60 feet), the temperature 

 being higher or lower according to the depth penetrated. 



3d. In regions of mountain corrugation thermal springs are more 

 numerous and of higher temperatures than in undisturbed regions, 

 probably because the mechanical movement attending j^lication in- 

 creases the amount of heat, and the fractures caused allow the water 

 to penetrate to greater depths. 



4th. Thermal springs.occur pre-eminently in regions of eruptive rocks, 

 whether recent or ancient, and no boiling springs are found outside of 

 volcanic areas, proving that in such areas the downward increase of 

 heat in the rocks is greater. The heat increases with the depth usually, 

 and yet may also be, to a certain extent, independent of the depth, 



CHANGES OF TEMPERATURE. 



From the fact that the existence of the hot-spring region of the 

 Yellowstone National Park is a recent discovery, no data of value can 

 be presented bearing upon the question of change in the temperature of 

 its hot springs. Temperatures have only been recorded for a few years, 

 and no changes outside of those due to the personal errors of observers 

 or differences in the thermometers used can reasonably be looked for.* 

 Lava and scoria are bad conductors of heat, and where there is a con- 

 siderable thickness of rock the heat in the lower portions may be re- 

 tained for ages. It is possible to walk over lava which is red hot a very 

 short distance below the surface, and which will ignite sticks thrust 

 into cracks in it. The non-conducting quality of lava as to heat is 

 strikingly shown in its preservation of ice in Italy and other places. On 

 Etna, in 1828, a great mass of ice was found under a covering of lava 

 which was the cause of its preservation.t The city of Catania, draws 

 its supply of ice from this source. Hot vapors emerge from between 

 masses of ice and the crater of Etna. 



During the eruption of Vesuvius in 1872 masses of snow were covered 

 with a thick layer of scoria, and afterwards by a stream of lava, and 

 three .^ ears later it was found consolidated into ice, but not melted.^ 

 Deception Island, in the South Shetland Islands, is composed of alter- 

 nate layers of ice and ashes.§ 



We can therefore readily conceive how heat may be retained in vol- 

 canic rocks, especially when they reach to immense depths, although 

 active volcanic manifestations may have ceased to show themselves for 

 long periods. It is probable, therefore, that if we had a record of 

 temperatures of springs of the Yellowstone Park, extending back for a 

 long time, little change would be noticed. 



*The comparison of temperatures taken in 1871 with those of 1878 will be found in 

 the various chapters of Part I of this report. 

 tLyell's Geology, page 412. 

 t JudcVs Volcanoes, page 110. 

 § Lyell, p. 412. 



