130 REPORT UNITED STATES GEOLOGICAL SURVEY. 



bight will be about 6 feet in diameter at the base and 2 feet at the top. 

 The water comes up to the top "within about a quarter or an eighth of an 

 inch. It is cold and muddy, containing exceedingly tine silt and gelatin- 

 ous mud in suspension. Upon watching the surface of the water an oc- 

 casional rise of a bubble can be seen, which disturbs it. Then a small 

 amount may flow over onto the upper rim of the cone containing it. 

 The moisture here deposited soon evaporates, but the mud and silt re- 

 main. In this manner the mounds are formed. If they attain a height 

 of more than 4 or 5 feet, the upper opening begins to close, and finally 

 nothing remains but an accumulation of mud. 



This mud is entirely destitute of vegetation, and exceedingly loose 

 when dry. An animal will sink in deeply while passing over it. Springs 

 were seen, containing a water-opening of only a few inches in diameter, 

 but one was found which measured fully 30 yards. Varying from the 

 others, the water was here not exposed upon the surface, but a film ot 

 fine mud had formed over it. It was found that the consistency of the 

 mud mixture was sufficiently great to sustain a crust of several inches 

 in thickness. Any object weighing enough to break through the cover- 

 ing could be throAvn upon it and would immediately sink out of sight. 

 We were not prepared to take soundings, but established the fact that 

 many of the cylindrical tubes containing the mud- water were more than 

 10 feet in vertical depth. At least for that distance an old lodge-pole 

 which we happened to find could readily be introduced. Breaking open 

 .a small mound, which showed an aperture of about 3 inches in diameter, 

 it was seen that the cylinder is lined with a coating of very fine blue 

 mud, about half an inch in thickness. The question soon arose, w r as the 

 water held in its position by hydrostatic pressure, or was an accumulation 

 of some kind of gas the cause of its rise in the tubes. A rifle-ball shot 

 down vertically into one of the openings produced a sudden eruption of the 

 whole mass. Water and mud were thrown to a height of about 10 feet, 

 covering the luckless experimenter from head to foot. From, a safer 

 distance the trial was several times repeated and almost always followed 

 by the same result. After everything quieted, the water was found to 

 stand from 2 inches to 8 inches lower in the tube than before. Crude as 

 this test may be, it shows the presence of gas at some depth, held there 

 under mechanical pressure. Probably some chemical decomposition fur- 

 nishes the gas, which has given rise to the formation and perpetuation 

 of these springs. We estimated their number at about 400, covering an 

 area of about half a square mile. Innumerable bones of animals, who 

 here sought to quench their thirst, prove the treacherous character ot 

 the soil. Frequently the surface, which appears perfectly dry and solid, 

 may be shaken by the foot for more than 20 feet. The entire place is 

 undermined and filled with muddy water, so that it requires some care 

 to get over it without mishap. None of the mud springs that have here- 

 tofore been discovered seem to possess exactly the same character as 

 these, nor are they so extensive. 



Green River Group. 



As indicated in Section XVI, the Green River beds follow directly 

 upon the Wasatch series. Packer's Creek flows in a southerly direction 

 into Bitter Creek, a short distance east of Rock Springs. West of it we 

 observe a high ridge composed of the light Green River shales. A gentle 

 dip to the westward is soon obliterated and the strata continues very 

 nearly horizontally. This slight inebnation continues for a long distance 

 westward. Early erosion has removed large masses of the strata be- 



