90 



GEOLOGICAL SURVEY OF THE TEREITOEIES. 



of thud is, of course, produced by the escape of the sulphureted hydro- 

 gen gas through the mud. Indeed, there is no comparison that can 

 bring before the mind a clearer picture of such a mud volcano than a 

 huge caldron of thick mush. The mud is so tine as to have no visible 

 or sensible grain, and is very strongly impregnated with alum. For 

 three hundred yards in length and twenty-five yards in width, the val- 

 ley of this little branch of Alum Creek is perforated with these mud- 

 vents of all sizes, and the contents are of all degrees of consistency, from 

 merely turbid water to a thick mortar. The entire surface is perfectly 

 bare of vegetation and hot, yielding in many places to a slight pressure. 

 I attempted to walk about among these simmering vents, and broke 

 through to my knees, covering myself with the hot mud, to my great 

 pain and subsequent inconvenience. One of the largest of the turbid 

 springs has a basin with a nearly circular rim 20 feet from the margin 

 to the water, and 40 feet in diameter. There are two or three cen- 



Fig. 26 . 



^^.^ 



'6 ^^j iL_1.-4= ^ 



?eet 



Jflog_ 



SULPHUR AND MUD SPRINGS, YELLOWSTONE RIVER, 6 MILES BELOW THE LAKE. 



ters of ebullition ; temperature, 188°. We may say, in conclusion, in re- 

 gard to this group, that while there is a great deal of activity in the 



